After two tries of finally doing it the right way, I finally defeated the Captain and got the Predator achievement. Now I am at Clemons (who technically I already defeated once on this file earning the Black Magic achievement which saves automatically even when Autosave is off), but I'm redoing the fight (Theoretically I could just go back in with Vera and Roald now that I technically have the achievement for defeating Clemons alone, but I seem to have developed a good enough 1V1 strategy in my opinion that bringing the others to the fight would probably take the fun out of it) to get Shandor's Darkstaff (I keep getting the other 2-handed weapon).
My 1V1 Clemons strategy for Cold Hydro runs is as follows:
This is similar to the strategy on the first page, but there are probably some differences as the one on the front page assumes that you're using your teammates.
Have two each of Flash and Mind Freeze (maxed out for best results), one Restore (also maxed out for best results), one Subversion, one Vapor Cannon, and an Ice wall (mine wasn't maxed out, but you might want to have it maxed if possible) in your combat wheel.
From full health to around 12,000 HP, Clemons will cast Unknown Condition on Sonny. After he does this, cast Mind Freeze. His attacks for the next 3 turns will heal you instead of hurt you (though I've noticed a glitch with UC where Clemons' first attack after being Mind frozen will hurt you instead of heal. At the very least, the damage number is in red font as though you got hurt instead of healed, but this glitch isn't incredibly frequent and any damage you did take will be healed next turn). While the Mind Freeze is active, fire the Vapor Cannon at him. Right after Mind Freeze wears off, or on the turn before, Flash freeze him. On the next turn I usually cast Ice wall. Then repeat until Clemons gets down to around 12,000 HP, at which point he stops casting Unknown Condition and starts casting other shadow elemental attacks on you, but those can be easily disrupted with Mind Freeze. I still cast Flash Freeze when the silence wears off (sometimes casting Subversion to recover some HP one turn during the silence), and cast Restore when Clemons is stunned, especially if Ice Wall is active, since that boosts healing effectiveness as well as defense. Repeat this until he gets down to the point where you have to stun him and cast Subversion (though I find myself screwing up the timing by casting Flash Freeze and Subversion a turn too early). If you do screw up the timing like I generally do, just hang in there and freeze-lock him until he dies, which is easier than his previous form since the damage he takes in his other form is doubled. Once he goes down, the Black Magic achievement is yours if you haven't already gotten it in another file.