This trophy is an absolute pain but goddammit I wanted to show my fanboy love for this series and I persevered through the offline gauntlet. I wish there were more random people online for matches instead since I never play at the same time other lobbies are open.
In any case, I got bored using Y'shtola at some point so I just went and leveled most of the other Marksmen to Crystal A this way. Other decent alternative choices are: Ace - spam ground - decent damage, the ball persists a bit
Kefka - spam Thundaga in the air , hold it until charged - excellent damage, very short burst fire
- combine the above with ground , hold it until charged - small but consistent packets of damage that add up quickly and last for a while
Yuna (if you have her) - spam both ground and air - exactly the same attacks with different visuals, great damage and the timing is just right for the AI to dodge in and out of the bubble for damage
Terra - just the default is fine, you can also start off with the initial charged up dash for extra starting damage
I would not recommend Ultimecia at all, since the damage is really low without Maleficarum boosts and you can't be bothered hitting the AI and wasting more time.
As a general rule, dash immediately towards the core bubble, stay around 3-4 steps away from it and spam an attack. Position yourself so your attacks are in between the core and the AI that's chasing you. That'll make the AI constantly dodge away from your attacks, typically away from the bubble as well, leaving the core open. Abuse high positions if possible, like in the XI stage, and the AI will be even more confused. Have at least one Vanguard on your team and a Specialist or Assassin in the other slot. Garland and Ramza were most successful for me, but YMMV.
I got mine at P. Lvl. 247. Good luck! You can do it!