I notice people on the board and in real life mentioning that the conclusion of Xenogears was the hardest fighting ever, or at least the hardest RPG fighting. This will probably spoil just a little, but I'll keep it to a minimum. I know the secret. It's magic. And expensive. But maybe it can help people.
First of all, get 3 GNR500's for Fei, each of which give you 50% increase in engine output . Second, get Fei the Omega 100 engine, which gives him only 1000 fuel, but an insane amount of output, therefore giving him insane damage. For armor, I recommend the Z alloy 800/30... It's 800 normal AR and 300 ether AR. Yes, 300. Equip it and see for yourself. It doesn't SAY 300, but the stat will increase by 300 all the same. These items can be bought in the lighthouse from the merchant, for a lot of money. And now, give Fei's character (not gear) speed shoes if you have them. They give his gear the haste effect when he wears them while piloting. You cannot buy them anywhere. That way, you can ignore booster. Now, grasshopper, you are ready.
Get to the end battles. Easier said than done, but www.gamefaqs.com has excellent strategy guides. One of them may have done my trick already. If so, know that I did this on my own, and do not mean to steal from anyone. Anyway, go straight to the fight with [insert boss name here], and ignore his four "supports." Make damn sure Fei is ready. Then waste the sucker. Just hit him repeatedly with Fei and he goes down like a sack of potatoes. No need to heal. Fei kills him too quickly. Then, just let Fei attack constantly for any further battles you may or may not have to fight. Hope that helps anyone having trouble, didn't ruin the game for anyone, and didn't take too long.