Well...lvl really only matters for the rune slots and only up to a point. Power is not terribly relevant.

So onto an actual tip. Back before I got a couple of lightbringer sets, I would typically set up my formation so that Cianna, Calvin, Shayd were the last 3 standing. An example of this would be:
empty-empty-empty
empty-damage dealer-damage dealer/thucy
shayd-calvin-cianna

I won't say that it is the best formation to use it because it probably isn't (I'm not going to do everything for you nor do I claim to be the best at banish).

Another thing is that you need some hit rating, a lot on shayd if possible. Then focus on increasing attack stats.