Wwll my strategy is this:

Use a team of Rikku, Wakka, and Tidus. Level the hell up til Tidus learns Hastega. Have OD ready, mainly Wakkas Attack Reels. If for whatever reason you dont have it, then get it!!!!

After this, if you use other characters, rotate them in for 1 turn to ge tthe exp. Then put the 3 formentioned in. I dont worry bout poison Breath, since he falls usually before he can use it.

Have Tidus cast hastega, then his next turn slow on Evrae. At this point, 1 perfect attack reels will kill him or at the least, bring him down to mear death (assuming you use him at all). If he uses Poison Breath (he will inhale) you can either try to move away (and then feel swooping scyth or whatever it is called) or have Rikku use an al Bhed potion to cure it. He shouldnt be too hard.

Also when levelling, have rikku steal every turn, especially the bombs to be able to customize armor with fireproof, useful in the calm lands and beyond.