Well.. there's the old standby, crystallize. Get two of them, cast them on him and keep him frozen while you wait for your debuffs to wear off and to heal. When you're ready, let him unfreeze and hit him a few times, then freeze him again before it starts to get dangerous. I don't remember if you can get a viable speed build by lvl 15, but if you can then that would be good because with that you could deal major damage the turn he unfreezes (the end of your speed combo) and then immediately refreeze him.
I got the black magic achievement using Hydro, though; this is just how I approach tough single enemies in general as bio, not specifically Clemons. When I beat Clemons as bio I had teammates, and just kept casting Disrupt to prevent his debuff.
