I was having trouble with that too the first few times, he was the first thing to sucessfully kill me lol. I'd been trying to tank it out with the Combat Clinic paradigm, but it usually wasn't ending well, because I was using hope as my secondary healer, and he usually didn't survive it. If he did, he usually got lit up like a christmas tree by Baptism in Ruin shortly afterward. So finally, I switched my party up again o be pretty much all out offensive. When he started charging destrudo, I just switched to Cerberus and pummeled the crap out of him, if you can get him to flinch, Detsrudo ends up being weak as hell. If you're still losing your party leader to this move, try setting Fang or Snow or someone with ridiculously high health as your party leader for the fight. Then if worst comes to worst, you can use Renew or a Summon to revive the fallen.