Best level 5 units

[Zechnophobe]

For me, Red Dragons are the best all around. They do amazing damage, and have significantly better health/leadership and damage/leadership than black dragons. Getting red dragons early on in the game is the single best boost you can get (Yummy shield of rekross!)

Bone Dragons however have their share of tricks. Eviln obviously, but also the target/timeback combo:

Step 1: Move bone dragons into middle of group of bad guys
Step 2: Cast target on them, and wait until end of turn
Step 3: At end of turn (Surrounded by attackers) use poison breath to damage many many stacks at once.
Step 4: Wait with bone dragons turn 2
Step 5: At end of turn, cast time back, to return them to full health, and fly into center of old spot, and use Poison Breath again to once gain hit a whole lot of stacks.

42 mana, second round of combat and you are at full health, and have done a whole lot of poison damage to a whole lot of units. Works just as well with allies. Take a bunch of archers to go in with bone dragon to help pick of baddies, works pretty good.