As a first-time player of this mod (and having read the comment about this before I got to said cave), I had no issues with the Primordites. Did what I could to gear Celes for Magic, and while she had to ST the spell, Ice did kill them in one hit, was hitting around 190-210 damage no problem. Now, it's not like it was a "hard" magic gearing, that early in the game, just whatever Terra had been wearing, armor-wise, along with the Magic Box relic (the +50% MP one) and whatever other relic could boost her Magic higher.
Beyond that, I am thoroughly enjoying this revamp to the game's mechanics, as I am about to be tackling the Floating Continent. The fact I've had bosses (and even some random encounters) put me down, is actually refreshing, forcing me to learn how to handle what is going on in the fight. Only "abused" save states right before bosses, simply to save time trekking back to them from the last save point in case of a failure.
Also, through pure accident, I think I overleveled a bit... Is 20 what you should be hitting the FC at?