If you played Heroes III before, you will notice preserve is a combination of Rampart and Conflux, this town largely depends on nature magic, and has 16 types of creatures, and you will be able to recruit all of them in town.
The two main types of heroes are druid and archer; a druid is absolutely necessary for a preserve town, since preserve largely depends on nature magic, it is recommended to have two heroes in the beginning. A druid should concentrate his/her skills on nature magic skills instead of summoning skills, in this way, you will be able to get high level nature magic early, this will greatly aid your combat. While an archer should concentrate on Archery skill, a grandmaster Archery skill can do a significant amount of damage. When both heroes are about level 8, you can separate them and team them up with another two heroes. An archer can also learn tactics, life magic and chaos magic to support the army, Learning Tactics will be very nice since Preserve units are quite fast, with tactics some of them can reach enemy within one turn.
There are three special town structures in Preserve, they are: Rainbow, Scared Grove and Creature Portal. Compare to other towns, the only special structure worth mention is Creature Portal, this structure gives you access of different types of nature creatures, including some of the best creatures in the game, such as Waspwort and Mantis. The requirement of Creature Portal is either Griffin Cliffs or Unicorn Glade. However, you should make every effort to build this structure as early as possible.
The requirement of Preserve Structures are generally high, wood can be a big problem in the first three weeks if you choose to build Elves' dwelling, while mercury, gem and crystal can be a problem later on. So planning for your resources is always the most important thing.
Below is a list of all nature creatures in the game, when comparing two creatures, it is important to remember all creatures have their own usefulness in certain situations. And depends on the map, make a wise decision; don't just stick on only one type of creature all the time, especially in Preserve you can built Creature Portal, which allows you to recruit other types of creatures to compensate your initial lose. Keep this in mind, having the best creatures doesn't mean you will win the game, attack early and balance your ranged/melee units are the most important things.
Level 1 Creatures
Sprites: Sprite is my favorite 1st level creature. They are very weak, but their movement is very high, and with no retaliation ability, they are probably the best hit-run unit. Sprites are extremely important in the beginning, however, because they only have average speed, you should always cast 'Speed' spell when fighting against some high-speed creatures such as bandits. In this case they are probably the best unit in the game to attract enemy's attention (although this needs some practice), meanwhile you can finish enemy off using your Archer and elves. Also, a sprite is a prefect scout, you can use them to collect resources and snatch artifacts around monsters and quickly run away.
Wolves: Wolves are very fast creature with high speed too, they attack twice, but they are quite weak and never done too much damage, the best strategy of wolves should be using them as decoy, but generally they never survive after any major battle.
Leprechaun: These Irish monsters are truly weak, never done much damage, even in a large stack, but who said good units should always do a lot of damage? Their best bet is casting Fortune, since good luck makes you take 50% less damage, it is indeed very nice if you separate them into several slots, and cast Fortune on your major units, afterwards use them attract enemy's attention, I found them very useful in any battles.
Level 2 Creatures
Elves: Grand Elves are gone, instead we have ourselves these female elven warriors, but they almost that good (they only have 18 HP now), they can shoot twice and other special abilities make them one of the best shooters in the game. However, the problem of getting elves is Treetop Lodge requires Ranger's Guild. So totally you need 26 woods to get them, this is quite expensive indeed. Acquiring them means you will slow your development down for at least 4 or 5 days. In some tough medium and small maps, if you want to attack early, you should go for White Tigers, since you will be able to get some good shooters later on once you build the Creature Portal, for example: Wasworts and Water Elementals. But in a large map or some maps rich in resources, make sure you get them.
White Tigers: White Tigers are very good 2nd level melee unit, they are fast and have first strike ability, very powerful fighter, they are also easy to acquire early in the game. The only problem in the combat is, because of their large stature, they usually blocked by some small objects such as bushes and rocks, this can be quite a problem, make sure your White Tigers are on the front line.
Satyrs: They had too much wine and women! But they are good and fun to use. They cast mirth, if you have them in several slots, and within next few turns all your troops will act before anyone else. They have even more HP than White Tigers, makes them good melee fighter too. High speed and movement also make them deadly foe, however, it is impossible to get them from the town in the beginning. And it is not worth it to buy them from Creature Portal later on. It's a shame they never act a major role in later battles.
Level 3 Creatures
Griffins: Griffins is the best 3rd level creature in Preserve, their unlimited retaliation is valuable, plus their high HP, speed and movement, if you can cast Snake Strike on them, they can be really powerful. In siege combat, a large stack of griffins can really mess enemy castle up. They are really lovely, except, that is, to build Griffin Cliffs you will need citadel, which is very hard to build in the first two or three weeks, so most time I would rather go for unicorns to save valuable time. Same as Elves and White Tigers, depends on the map you are playing, in maps rich in resources, make sure you build Griffin Cliffs.
Unicorns: Unicorns are weaker compare to griffins; their blinding ability would be very nice if the chance is not random, plus they cannot fly, made them quite stupid in the combat. But good thing is, you can acquire them with a fairly cheap price, and your town development can be a lot easier. Their growth is also slightly higher than griffin, if you want to strike early, go for Unicorns. Make sure Unicorns are on your front line, since they are quite big.
Waspworts: Waspworts can be very good if their speed and movement is higher, but they are almost that good, they have 60HP, attack and defense is the highest amongst all 3rd level nature creatures, plus their 100% weakness and ranged attack, make them a nightmare on the battlefield. And best of all, they have a high growth; it is very easy to gain a large stack of waspworts. Take them if you don't mind to slow your army down, they can be really helpful. Also, when defending a town, they are really great.
Elementals are great units in some certain circumstances, and they are great against death/chaos creatures (Vampires' life drain for example), but in general, because you gain 3rd level nature magic quite early, they are not worth to take from Creature Portal, but if you summon them during combat properly, you will find them are quite handy.
Air Elementals: Air Elementals are fastest among all elementals, also, they gain 100% extra defense against physical attacks. You will be able to recruit them in your Creature Portal, but since there are better creatures out there, they are not really worth to take.
Earth Elementals: Personally I think earth elemental is the worst 3rd level creature due to there extremely low speed, they can be quite good if you summon them during combat and protect your weaker and ranged creatures. They have 50% magic resistance, which means most enemies will attack them physically; this made them perfect cannon folder. But don't take them; they slow your army down.
Fire Elementals: They are average 3rd level creatures due to their ranged attack, and they also have average speed and HP. A large stack of fire elementals can do significant amount of damage, fire resistance comes in handy against chaos magic, but in general, they are not worth to take.
Water Elementals: What's so good about this creature? They have relatively low speed and movement; HP is lower than some 2nd level creatures. Cold resistance is almost useless in the game, but they have a few choice to cast spells, so separate them into several slots, and cast spell to slow targets can be quite useful, their Ice bolt never done much damage unless in a large stack. But in some situations you need magic, water elemental is your best choice.
Level 4 Creatures
Faerie Dragons: Faerie Dragons can be either very good or very bad; since there are just too many creatures in the game have magic resistance. Although their magic mirror ability is really scary, the initial spells still affect them. They are prefect unit against magic, but when fighting Black Dragons, they completely become 3rd level creatures. So dependents on your enemy, you should make a choice between Faerie Dragons and Phoenix, since Faerie Dragons are just too weak in some circumstances.
Phoenix: Phoenix has extremely high movement and speed; they always act faster than anyone else. Their area attack comes in handy too, since you can always attack the weaker unit and hurt another stronger unit in the same line. Rebirth is also a very nice surprise for your enemy. I personally recommend taking Phoenix as your front-line soldier instead of Faerie Dragons.
Mantis: Now these guys are really worth to take from your Creature Portal, they have extremely high attack and defense, they can fly, they have binding ability and first strike. If you also cast 'Dragon's Breath' and 'Speed' on them, they can be truly powerful on the battlefield. They have similar stats as Dragon Golem, but because they are very mobile on the battlefield, they are much better than those machines. Just one problem though, their growth is quite low, and they are way too expensive, but if you are rich, they just won't let you down.
Use your elementals
Although taking elementals from your creature portal is not a great choice. In tough battles, summon them is always a good idea. Elementals are great due to their different special abilities. Depends on the situations. Summoning elementals is also very good in siege battles. For example: use your Water Elementals to cast Ice bolt to attack enemy's ranged units, especially when they are standing on the shooting tower; Or use your Earth Elementals to block missile and magical attacks.
Another example, you can use your Air Elementals as decoy in the siege battle. Send them into the enemy castle next to the gate without attacking the gate (in this case AI won't come out and attack you), because their physical toughness, they will most likely survive the enemy attacks. While all enemies are attacking your Air Elementals, you can use your archer and elves to finish enemy off.
Quick Sand is a powerful spell
Don't forget you can link several quicksands together; this is great when fighting wondering monsters. Especially when you are at low levels. Several Water Elementals will also do the trick. In narrow terrains, this spell is even better than mass slow.
Put your druid in the front line
Unlike other magic user, druid should always in the front line (except in the beginning). This ensures your summoning spells have the maximum usefulness during combat. Here are some formations.
Front line: Druid/White Tigers/Phoenix/Unicorns
Back line: Sprites/Wolves/Archer
In this case you decided to strike quick, so you go for white tigers and unicorns. White tigers and unicorns in the front line can make a front assault, sprites on the back row due to their high movement, while wolves act as a protector of druid/archer hero. This formation is good for protection of your heroes.
Front line: Druid/Faerie Dragons/Wolves/Griffins
Back line: Elves/Sprite/Archer
This formation provides very mobile movement on the battlefield, it is also a very nice hit-run formation, the druid hero should starts summon creatures from the very beginning because faerie dragons also need protection.
Front line: Griffins/Phoenix/Mantis/Druid
Back line: Wolves/Sprites/Elves
This formation concentrates on physical attacks, also effective in a quick strike, wolves and sprites provide protection for druid so the druid will have chance to cast protective spells in the first few turns.
Front line: Sprites/Faerie Dragons/Elves/Druid
Back line: Wolves/Water Elementals/Griffins
This formation is only good when fighting large stack of slow enemies. Elves in the front row to get a lower penalty, Sprites may use hit-run tactic more effectively. While water elementals were put on the back row due to their magical attacks. And griffins and wolves provide defense.
However, there are just too many situations you will encounter, keep in mind changing formation can indeed change the winning side of a battle. Generally: melee units should be put in the middle, and non-fly melee units should be put in the front row. And ranged troops should be put close to a druid or a well-protected area. As wolves, due to their low HP, they should be treat as protective unit in most situations.
Speed/Mass Speed: Very important in any battles, if you have the chance, make sure you cast this one first.
Giant Strength/Dragon Strength: Giant Strength is a very good boost of HP and damage in the beginning, while later on you might want to cast 'Dragon Strength' instead. Try to cast these spells on your main hero to gain an extra life, or you may cast it on your main units.
Fortune/Mass Fortune: Give 100% good luck, which makes you take 50% less damage. Very nice, but keep in mind when enemy attack you magically, this spell won't work. If you don't have Mass Fortune, you might want to cast 'Summon Leprechaun' instead.
Quick Sand: As mentioned before, very useful in narrow terrains.
Snake Strike/Mass Snake Strike: Make sure this spell is cast on griffins, but don't cast them on white tigers (n/t).
Anti Magic: It's actually not that useful since if you cast this spell on a hero, you can't even drink any potions, but casting this spell on your units to prevent genies' create illusion is a very nice trick.
Necromancy Ward: Not truly effective in the later stage since enemy would rather concentrate on using offensive spells instead of raising undead.