there's no real way of knowing u just sorat get a feel for how high your levels should be. as for levelling al; chars, they only need to do 1 move in a battle to get exp, so u can send em in, press triangle (this wil make them guard and their next turn will come round quicker) then switch em for someone else. i usually kill all the stronger enemies first so there's one weak one left before doing this.

u can revive aeons by checking a save sphere. they also revive after a certain number of battles. this is different for every aeon tho. the stronger they are, the more battles it takes to revive them, i think valefor is like 8 battles n bahamut is like 24!?

as for the fite against seymour, ur hp levels should be around 1500 or so (except rikku cos hers r pants anyway).