I found jecht / braska's final aeon pretty easy 2 be honest

he only has hp 60000 in the first round and 120000 in the second

Just get as many celestial weapons as possible, get all of your charecters and aeons to full overdrive guages and let loose

If that doesn't work then u shud train some more
(mail me 4 details)

Oh and dont let him get his overdrive : use talk when hes going 2 use it, it will lower his overdrive