The way I see it. Even if serenity lives, the only way Jack will stand a chance is if I amp him greatly (which I wouldn't recommend cuz fighting op with more op just leaves more problems) or give him equipment that exposes the living hell out of everyone's weaknesses. Now while that may be a good obstacle for a team to overcome, it is simply extremely boring for a person to rp like that. 

Makarov and Tidus both even stated the extent of how op they were.

The only logical step is for me to leave.