Also, please post your strategies if you remember them and what happened to you.
Me, I used this strategy, but modified it a little so I wasn't copying it.
http://www.gamefaqs.com/console/psx/file/199365/50268Also, I didn't want to go all the way back to the beginning of Disc 2 to get more evasion rings (since that's the last time I saved in the other card where I would have access to buying evasion rings), so I had to make do with just one per character. I guess Evasion Rings accumulate? Instead of Billy, I used Emeralda and her spells, which did around 4000. I lost the first time, but the second time I boosted everyone and spammed Wild Smile b/c he kept occasionally hitting me for 9999 even with all my magnetic coats on. Weltall spent most of the battle with a few hundred HP, which was scary! I just had him do System Id and Kishin for about 5000 of course until his fuel got low, then I was going to switch to Big Bang, but he died before that could happen. Oh, he had the Trader Card. Meanwhile Bart was attacking a few times in order to try to break his ETH machine, which for some reason didn't work all the time, and at some point I realised it wore off when he finished off Emeralda. Unfortunately, all that does is reduce his Chi attack from about 4000 to about 800, which does help still. Luckily, he had mostly left Bart alone, so Bart had close to max HP left. I didn't know how much Alpha Weltall had left, but I had been going at it for a while. I decided to just attack for pitiful damage trying to break his ETH machine again but to no avail. Suddenly, Bart had only a few hundred HP left, so one more hit and he was dead no matter what! As Salieri would say, "And you know what happened? A miracle!" Bart managed to enter Infinity Mode and I did his Hyper X attack, which finished Alpha Weltall off, then I got the Slayer Robe. I would have liked to have kept everyone alive to have everyone get Exp, but Fei had just leveled up to 90 right after Deus, and it was so difficult getting the Slayer Robe, so I didn't care! I was too happy to soft reset.
EDIT: Whoops, what happened to my question on the poll? It's supposed to say "Have you defeated Alpha Weltall and obtained the Slayer Robe?"