On my third playthrough
1.) I killed Ultimate Weapon. I have no idea the trick to finding him on the map but I eventually saw him flying around, fought him twice before the final battle at Cosmo Canyon. But his dying move is SHADOW FLARE, which is guaranteed to kill its target unless you have M Barrier up, thus ensuring that one party member doesn't get the 35,000 experience!
2.) Morphed a Master Tonberry. Now is there a way to do that WITHOUT the right analog button? I hit him with a few Limit Breaks including Meteorain then kept using Morph on him for like a hundred turns while having to use a few Phoenix Downs to revive Cloud or Yuffie if he used Everyone's Grudge or Knife on them (somehow Red XIII could survive Everyone's Grudge).
(Actually make that three things - I accidentally killed a few of them with Limit Breaks, I've never actually killed a Master Tonberry before)
So all three party members have Ribbons now.
But btw, you can NEVER use Sense on Master Tonberries regardless of their HP (I kept having Cloud use Sense and it didn't work), so you just have to keep spamming Morph and guess whether he's getting low.