This trophy is still quite the grind even with this little exploit, so prepare accordingly. Sora's grind to level 99 is a cakewalk compared to Riku's because you need to actually play and pay attention to your cards instead of spamming Megaflare.
The method I used to get this trophy was by going to B1 Castle Oblivion because the enemies on this room offer the most experience and you have access to the best deck in the game. I'd recommend doing this once you have a maxed out AP and the difficulty on beginner. I haven't tested this method on the higher difficulties.
What you'll preferably want to do is to go straight to the second room with a low level map card (doesn't matter which one you use) and then immediately turn around and use a Strong Initiative card on the door you just went through. The reason you'll do this is to have close access to the free save room with Mickey in it. After you have the Strong Initiative room set up next to the save room, you're ready to start the grind.
In order for this trick to work you'll need to hit the enemy in the field to activate Strong Initiative's effect. When the battle starts you must IMMEDIATELY go to your Maleficent enemy card and start killing off the highest experience yielding enemies a.k.a the most dangerous ones. Once you are left with ONE lower tier enemy, FLEE from the battle. The heartless will respawn on the field and you'll be able to do another battle with it. These are the tiers of the enemies to defeat as follows:
4. Blue Rhapsody/Red Nocturne/Yellow Opera
5. Shadow/Green Requiem
The reason you want to take out the Neoshadows and Wizards first is that they are annoying to face, offer the most experience, and can prevent your escape from the battle by spamming their powerful attacks. Try to save Shadows and Green Requiems for last because they offer the least experience and are easier to escape from (Green Requiems don't even attack). Make sure to SAVE after you level up and try to remember the enemy group spawning patterns. It's hard to really explain in text, but you'll understand it by yourself eventually because there's only 5 or so patterns they spawn in. That and you're still going to be on a long grind even with this trick. Here's a quick TL;DR in case all this was too much to take in:
1. Set up a Strong Initiative card next to any save room in B1 of Castle Oblivion.
2. Hit an enemy on the field and immediately go to your Maleficent enemy card.
3. Defeat all strong heartless and leave one weaker one (Shadow/Green Requiem) then flee the battle.
4. Rinse and repeat until leveled up.
Hope this helps out. It's one of the more tedious trophies in the game that is overshadowed by the even more tedious 6 playthroughs of the game required for the Platinum and the Card Master Sora trophy.