Jecht was a pain in the ass for me to kill in the first playthrough. It took me six or seven attempts. What I did was to disable the pagodas so that they stopped strengthening him and healing him. I used Armour and Mental Break with Auron to lower his stats and then used Lulu’s magic and physical attacks until he drew his sword out of his chest.
Jecht has a high power move once he does this which Tidus can talk him out of once or twice. It’s a trigger command that calms Jecht down. Bring in some of your Aeons (in Overdrive mode, if you can be bothered to charge up the Overdrives.) Hopefully this should be enough to defeat Jecht.
anyway, just try to keep it simple, I don’t bother to touch the pagodas, it just wastes a turn and makes things worse for me. Attack for more than 3000 damage so you can make a difference, and have auron use zombie attack, then he’ll only heal 1500 per turn