Pixie
PixieHealth: 3Attack: 2Shots: noneMovement: flying
25 gold (20)Hex size: 1Defence: 2Dmg: 1-2Spd: 7 (extra swift)
They are the weakest (if we forget peasants of course), but cheap and plentiful. If there are some sprite dwellings nearby, you can get several dozens straight away. Make good use of a wait command - pixies are fast and will outwait most weak monsters and then have 2 moves in a row. Pixies should only attack if they can kill the target because their 3 hit points and 2 defence make them an easy target.
 
Sprite
SpriteHealth: 3Attack: 2Shots: noneMovement: flying
30 gold (20)Hex size: 1Defence: 2Dmg: 1-3Spd: 9 (ultra swift)
Special: enemies cannot retaliate.
A great upgrade, worth doing early: for a tiny extra 5 gold sprites get that crucial extra speed, higher damage and most importantly, no enemy retaliation. Bless them, it may double the damage. Outwait the enemy and strike only when you're sure to kill or if you're able to fly back next turn before the enemy attacks them. Sprites are quite a strategic unit, weakness of their nice body is only a deception.
 
Air Elemental
Air ElementalHealth: 25Attack: 9Shots: noneMovement: ground
250 gold (6)Hex size: 1Defence: 9Dmg: 2-8Spd: 7 (extra swift)
Special: immune to mind and meteor shower spells, takes double damage from lightning bolt and chain lightning.
The best level 2 creature which can even beat some of the level 3 creatures! I really suggest you keep them for an upgrade. Needs to be blessed for a good attack. Otherways, an all purpose unit: equal attack and defence and a good speed. Their power is compensated by quite a low population and the weakness of level 4-5 conflux units.
 
Storm Elemental
Storm ElementalHealth: 25Attack: 9Shots: 24Movement: ground
275 gold (6)Hex size: 1Defence: 9Dmg: 2-8Spd: 8 (very swift)
Special: same as air elementals + can cast protection from air,
no hand-to-hand penalty (that doesn't actually work).
The upgrade makes it even better - 24-shot range attack, speed and even a spell that will protect allies from those lightnings, all for a crazy 25 gold extra! Storm elementals are amongst the best level 3 shooters at level 2! (along with ice elementals) Not much special strategy here, just keep them blessed for that full 8 damage. Make that upgrade early: army of storm elementals and sprites can take out most nearby monsters with minimal losses.
 
Water Elemental
Water ElementalHealth: 30Attack: 8Shots: noneMovement: ground
300 gold (6)Hex size: 2Defence: 10Dmg: 3-7Spd: 5 (slow)
Special: immune to mind, ice ray and frost ring spells, take double damage from fire wall, inferno, fireball and fire shield.
The most powerful level 3 creature, little more defensive than offensive. Needs to be blessed, but less need than for air elementals.
 
Ice Elemental
Ice ElementalHealth: 30Attack: 8Shots: 24Movement: ground
375 gold (6)Hex size: 2Defence: 10Dmg: 3-7Spd: 6 (swift)
Special: same as water elementals + can cast protection from water,
no hand-to-hand penalty (that doesn't actually work).
Good upgrade, similar to that of storm elementals, but do you want to pay 75 extra gold? They are about the same as storm elementals, but slower and have no weakness for that common lightning and are imune to second blast spell - ice ray... They are actually quite good because it's harder to just blast them like you can do to storm elementals. Still, I prefer to save 100 gold and buy a faster storm elemental with quite a similar stats. Best level 3 creature I guess (thanks to that magic resistance).
 
Fire Elemental
Fire ElementalHealth: 35Attack: 10Shots: noneMovement: ground
350 gold (5)Hex size: 1Defence: 8Dmg: 4-6Spd: 6 (swift)
Special: immune to all mind & fire school spells, take double damage from ice bolt & frost ring.
Ok, now the falldown starts: fire elementals are the weakest level 4 creature! They compensate with a little higher population though. I would rather recruit ice/storm elemental if I have a choice. Little more offensive than defensive, doesn't need bless as much as other elementals do and fire immunity is quite good - cannot be berzerked or cursed and won't take any damage from armageddon.
 
Energy Elemental
Energy ElementalHealth: 35Attack: 12Shots: noneMovement: flying
400 gold (5)Hex size: 1Defence: 8Dmg: 4-6Spd: 8 (very swift)
Special: same as fire elemental + can cast protection from fire.
Speed, attack and the flying abbility are well worth 50 gold, but they are still at the weak end of level 4 creatures. Still worth having though, now they are even more of an offensive unit.
 
Earth Elemental
Earth ElementalHealth: 40Attack: 10Shots: noneMovement: ground
400 gold (4)Hex size: 1Defence: 10Dmg: 4-8Spd: 4 (extra slow)
Special: immune to mind, lightning bolt and chain lightning spells, take double damage from meteor shower.
The weakest level 5 unit with a normal level 5 price tag... A little more population compensates, but nothing can compensate such a low damage except for a special heroes (Erdamon and Thunar) who make earth/magma elementals more of a normal unit for it's price and level. But still, with such speed I suggest you use them for garrison defence. By the way, always take them along if someone bugs you with powerful lightning, that immunity is very useful!
 
Magma Elemental
Magma ElementalHealth: 40Attack: 11Shots: noneMovement: ground
500 gold (4)Hex size: 1Defence: 11Dmg: 6-10Spd: 6 (swift)
Special: same as earth elemental + can cast protection from earth.
More speed, a little more attack/defence, better speed, better damage and a spell for an exta 100 gold, good upgrade I'd say, but still, unless you have one of those special heroes, magma elementals are quite a weak attackers. Now you can use them to protect the rest of your troops from implosion. If you're using ice elementals already, magma elementals won't slow you down any more.
 
Psychic Elemental
Psychic ElementalHealth: 75Attack: 15Shots: noneMovement: ground
750 gold (2)Hex size: 1Defence: 13Dmg: 10-20Spd: 7 (extra swift)
Special: immune to mind spells, attacks all adjacent hexes, no enemy retaliation.
Hydras once held a protest, but their special abbility wasn't copyrighted, so these guys retained it. But hydras still can brag about attacking 8 hexes instead of magic/psychic elementals' 6 though. Finally a really nice unit at level 6. Not that strong 1v1, but when it comes to crowded parties, these guys know what to do. Do take them along anywhere, they are quite cheap, and don't fear enemy retaliation. Psychic elementals are more offensive than defensive. Position them well, always adjacent to few enemies and this way after one of those attacks psychic elemental, he will retaliate to all enemies standing around!
 
Magic Elemental
Magic ElementalHealth: 80Attack: 15Shots: noneMovement: ground
800 gold (2)Hex size: 1Defence: 13Dmg: 15-25Spd: 9 (ultra swift)
Special: immune to all magic, attacks all adjacent hexes, no enemy retaliation.
Nice damage improvement, speed and few more hit points, but most importantly, immunity to all magic! This makes them excellent for the armageddon squad which can contain magic/fire/energy elementals plus firebirds/phoenixes who are all imune to fire magic. It's a really awesome upgrade for only 50 gold, especially that the enemy cannot blast them anymore. Keep in mind that beneficial spells don't work on them too, so if you're a big on beneficial casting, recruit them unupgraded. I would preffer total magic immunity to bless/prayer/shield though.
 
Firebird
FirebirdHealth: 150Attack: 18Shots: noneMovement: flying
1500 gold (2)Hex size: 2Defence: 18Dmg: 30-40Spd: 15 (super quick)
Special: immune to fire school spells, attack affects 2 hexes.
Good candidate for the armageddon squad, can outspeed over half of level 7 creatures. Somewhat weak for level 7 though, but the highest level 7 population (which is double of any other level 7 creature) compensates it well. Price is quite low too.
 
Phoenix
PhoenixHealth: 200Attack: 21Shots: noneMovement: flying
2000g + 1m (2)Hex size: 2Defence: 18Dmg: 30-40Spd: 21 (blazing fast)
Special: same as firebird + chance to cast expert ressurect when killed (if it does, stack of 1 will ressurect 1, 5 will ressurect 2, 9=3...)
Phoenixes always go first on the battlefield (unless someone with ballistics hastes an arch angel/devil on catapult's turn, hehe) and when they do, it's an excellent time for a first spell, and if all your army is immune to fire, why not make it a powerful armageddon? Phoenix is the best unit in the game to use for the armageddon squad. Upgrade is worthy. The ressurect ability works sometimes, so split phoenixes into proper groups to make resurrection more efficient if it happens: groups of 1 are the most efficient - there you have a chance for a second life for each phoenix, then add 4 on top of one each time, so the groups are: 1,5,9,13,17,21,25,29 (can you get that many?) etc. Their stats are now very low compared to upgraded level 7, but low price, high population and strategic usefulness makes them worthy.