Oh man... That is probably one of that hardest bosses in the game if you are not far enough into the sphere grid with your characters. What you want to do, is memorize Evrae's attack patterns. Then use the according character to take care of him. When you are keeping Evrae at long rang, which you should for most of the battle because I found him easier to take care of at long range, you use Wakka, Lulu, and whoever else to support those two while they attack. If you need to bring him in for close range, use Tidus, Auron, and another person to support them with healing.
You just have to keep at it. Eventually you will get it. My first time ever fighting Evrae it must have taken me five or six times before I finally beat him. The next play through I did of that game I made sure I trained like a beast and made that battle easy as pie. It has been a few years since I have played FFX, but that is the strategy I used. With a lot of bosses in that game, you just have to get used to their moves and try to predict when they will use a certain move. Then switch in the right characters. Good luck.
