Artifacts. Do It Yourself. (a tutorial for Academy players)

I almost always play as mages, using Hafiz. If you're playing as Hafiz and need to place a single artifact on a gremlin, choose either initiative or magic resistance! It's very helpful... And if you're going for a full mini-artifact set: For gremlins, I choose magic resistance/defense/initiative.
Other units have been well-described earlier:

Rakshasa: Initiative/Speed/Luck
Mages: Initiative/Health/Luck
Djinn: Luck/Speed/Initiative
Thunderbirds: Luck/Initiative/Magic Resistance