With Tidus: make him cast 'haste' or 'hastega' on YOUR party members
Try using moves like 'delay attack' and 'delay buster'.
Use 'zombie attack' and other status inflicting attacks. Believe it or not, that's the easiest way to kill the last boss: inflict him with 'zombie' and feed him a phoenix down: he dies (it's so easy, it is almost depressing).
What does your enemy look like? A plant. Try using 'fire' attacks; they usually work.
If it's an ice elemental, use 'thunder' attacks.
This is how the elements work:
Ice - Fire
Fire - Ice
Thunder - Water
Water - Thunder
If your squad isnt strong, don't make your aeons stronger to compensate for that. It will only lead to bad things. Trust me...
He uses his lance to cause 'zombie' on your party and then Mortiorcis (or something) uses 'full life' to kill you. I suggest customising armour with 'zombie proof'. It would greatly increase you chances of winning (Note: to do this you will need lots of 'holy water', and of course Rikku). He will use a move called 'cross cleave' (I think), which will attack all of your players. I had Auron in there, because he has a high HP-level. He could also cast haste on himself, so he could use a 'phoenix down' or a 'mega phoenix' to cure my players. Some people find it easier to attack Mortiorcis, but I used 'reflect' on Seymour, and attacked him with my aeon overdrives, so that way Seymour couldn't be cured by Mortiorcis.
