Chick, could you please tell me what exactly is the problem? A certain move the
machina does, or something?
Maybe you just need to apply the elemental rules and attack it with thunder-infused weapons, at that point you should have the respective weapons for both Wakka and Tidus. It is always a good strategy to use Haste (or Hastega, if possible) at the very first move you have with Tidus. Also use his Cheer a couple of times, and with Wakka, just keep hitting the thing. Use potions with Tidues, then attack.
If the attacks you recieve are devastating and the damage you deal is minimum, your level probably just sucks and you need to level up before going into this battle.
I hope this helps. Keep me tuned