Okay- here's the strategy I used:

First of all, Yuna, Rikku, and Paine all have to be Lv:99. Your Gunner has to have mastered her skills so she is able to get the most out of Trigger Happy. You also have to have Cat Nip, Speed Bracer, Mega Phoenix's, and the Mascot dressphere is helpful- but not required. So let's say you want Rikku to learn it. So you equip Yuna as a Gunner with Bloodlust and Wring to bring her HP into the critical area, then remove those and equip Cat Nip and Speed Bracer to her. Rikku will be the Gun Mage and make Paine a Dark Knight or Mascot.

Now I'll assume you've slain enough Tonberrys so that when you encounter this Mega Tonberry, he'll Oversoul. Make sure you save before the encounter just in case something goes wrong. MAKE SURE YUNA'S HP IS STILL IN CRITICAL STATUS AND IS EQUIPPED WITH CAT NIP OR ELSE THIS NEXT STEP WON'T WORK! Anyways- when you get into battle with him as soon as you can, have Yuna use Trigger Happy to quickly take out the little Tonberry and then wittle down Mega Tonberry's HP. Make sure you don't shoot him too much or you'll go right past the critical status and hit KO on him. If this happens, just reset your game. Rikku's Scan ability- since she's the Gun Mage- helps you decide on this. If he KO's any member of your party have another member quickly use a Mega Phoenix or Phoenix Down. Yuna is basically the one doing all the work while Rikku and Paine function as healers. If you've done this all correctly and the Oversouled Mega Tonberry reaches critical HP status, he'll almost instantaneously use Cry in the Night on your party and your Gun Mage will learn it. Make sure you heal immediately and kick its sorry before it's Game Over!

And there you go, my friend. My strategy. Hope it helps! Ask again if you have any more trouble and I'll see what I can do. :giddy: