You may or may not have decided "heh, screw it" when trying to get Ultima at Limberry. Or perhaps you didn't wanna deal with it in Limberry 1, and even less Limberry 2 cause stealing full Genji already drained all of your effort.

Fear not! While threads and stuff keep saying that getting it at Mullonde is suuuuuuuuper hard and random, the answer is: MORE RANDOM!

That's right, we'll control the fight and exploit Confusion! It's better to spend more time praying on Confusion's randomness than the AI's.

Source:
The last comment on https://gamefaqs.gamespot.com/boards/937312-final-fantasy-tactics-the-war-of-the-lions/42433445

This guide is purely a confirmation that it works really well!