In version 1.0, Lina with Ice Bolt is the most effective.
In version 1.5, Ice Bolt doesn't deal as much damage, so Reaper is the best. The effect of his time rewind ability isn't noticeable in the stats, but it's with his help that I won half of the battles. First, I put on the Glutton armor, second, I use time rewind, and the battle is won. Closer to the end, Soul Steal and Black Hole are more useful. Zerok is the weakest spirit; of his abilities, only Wall might be useful.