Henne Mines bats gave me about 3 million EXP per hour. Sleepga + 3 waves of Curaja wipes them out. Rinse & repeat. I took all 6 characters from 67 to 99 in about 7 hours total, over 4 trips.
On Yiazmat I used Arise for the first 80% of the fight, but once he doubled his level it seemed like I wasn't making any progress because of how long it took to get people back alive with Arise so I switched to Phoenix Down spam.
Oh and if you're 99, Masamune + Genji Gloves on your main DD is the way to go in my opinion. I had my Tournesol and Yagyu Darkblade on support characters and just Berserked Vaan. Spam Expose at the beginning of the fight and everyone hits for 9999 until his defense goes up, then they all hit for 6999.
The next day I beat Omega and then left the arena and overwrote my save file not realizing he actually DROPS the Omega Badge, unlike every other boss/major hunt mob in the game. I managed to screw myself out of the Wyrmhero Blade, so I just finished the story and packed up my PS2.