Also for the Dungeon, use a set of Reds and have them stand back while you take the damage.
Once you do, build up a set of armor with high defense using a lot of Browns.
Don't bother with the Dungeon until you get the third forge in the game.
As soon as it's gone past you, send 5 or 10 greens at it and turn on the minion boost spell at the same time.
To kill a unicorn: Let it charge you and dodge it.
The first level minion boost spell is probably the best spell in the game, since you retain total control of your minions like normal.
A green minion who's attached to a creature will do a lot more damage than an attached brown, a green minion who's not attached will do a less damage than a brown who's not attached.
If a creature is on fire he can't fight and he'll take more damage, so having a few reds around is always helpful.
Prioritize unlocking up all four different types of minions first, it helps cut down on back tracking.
Make sure they don't get attacked though, because they die really fast.
Put all your blues on a flag near a fight, they'll automatically run out and resurrect any minions who die.
Maxing out the best set of armor in the game (Arcanum wasn't it? I forget) with a high HP regen stat lets you solo the tower's arena pretty much by yourself (which is almost necessary for some enemies like the giant slugs). It'll let the Overlord rush into battle at times as well and you won't have to worry about the consequences since he'll just heal himself.
Dump enough minions into the stat on your armor that gives you at least a small amount of HP regeneration, and keep doing it over time.