[Chaos]

1. They do ridiculous amounts of damage.

2. If they can survive for more than one turn, they double that ridiculous amount of damage.

Summon storm is also equally horrifying with a plethora of high level youkai because stacking high amounts of FP is childs play.

Youkai are also quite fragile. A good number of them can be utterly destroyed in a single round, not to mention that one of Magic Gunner's bullets can de-summon Youkai quite easily. If Fast Offense didn't exist, you'd be hard pressed to see a Summoner actually summon.

As for the OP, I think (Youkai LV*2.3~2.4) would be a fairer number, given that you still have to grind a full level track for it and the spells don't receive any of the benefits that other spells enjoy, i.e. Evoker's boosts. The other suggestions are completely fine and should be implemented.