I'm stuck on the battle where you're on top of an airship headed to the city of Bevelle to interrupt Yuna's wedding and then this giant wyrm named "Evrae" appears and I can't beat him... hints and tips please?

This is also what I used in my strategy. This is a good way also. Listen to him.Cast haste on Tidus then slow on Evrae. The key is to time Cid's movements of the ship using Rikku and Tidus to avoid the poison attack. Try using cheer to increase damage but the most effective way to take it down is to be as close to it as possible so Auron and Tidus can finish it off.
Just check who is nearest to it in the Sphere grid. Everyone at one point can learn every spell. I got so freakish on X that I maxed out my entire sphere grid for each character.wait, who can learn slow?
Yunalesca was hard, but I actually had a harder time with Seymour on Mt. Gagazette (sp?).I think i just beat the **** out of it with wakka then moved it in and used tidus and auron. Haste is good. And alot of cure. That was prolly second hardest boss for me. Yunalesca being the hardest
That is partly why this battle is so hard. At this point most people only had Yuna as a healer. Which leave Rikku to do the healing with her mixtures, which is actually quite handy.it sucks how I taught Yuna Cura.. nek minnit shes getting married and one of my only ways of healing is to use either Mega Potions or Al Bhed Potions