A good strategy for this is to use Sonia as you leader character and fight an Orc. You can find one early in the game in the Ladlum Caverns
area - the second section called Howling Hole
In the top left of this area you'll notice a large red dot which indicates the Orc. You can try to tackle it the first time you come through her (in chapter 2) or come back later. I went back around level 25 in chapter 4 and the Orc had level scaled so I could still pull off the 99 combo before killing it.
As Sonia, you want to mash Y to use her Break Attack until her AP runs out (indicated by the green ring emptying). Then immediately use the Million Thrusts
Force attack. Repeat this combo as necessary and you should be able to build up the 99 combo. If you mess up, simply exit this map area and come back and the orc will respawn.
Here's a video of the method. All credit to the creator "Gaming With Abyss".