Try leveling up a bit if you want. Make sure all of your aeons are on overdrive before the battle. On his first form just attack him as you normally would. Don't summon too many aoens though, just summon either one or two of your weakest ones (save your stongest ones for the second form) or just have your characters hack him away on the first form. On the second form, make sure to use all of your characters overdrives. When Jecht is about to go on overdrive summon an aeon to block it. Then use your strongest characters to finish him off and that should be it. Oh and try to equip your characters with armour, if I can remember I think he can petrify you, but I'm not sure. good luck!