Also on the subject of Miguel, SealAll doesn't stop enemy tech skills. So even if it did work (or AntiWhite), he'd still unleash HolyDragSwd on you. Given, if he hasn't been defeated by the time he resumes his normal pattern (after HolyLight/MeteorShower at low HP), something is likely wrong.
On topic though, I'd like to add the standard "upgrade your equipment" and "allocate a lot of HealAll elements" strategies to the list. If you are still having trouble, abuse MagNegate and PhysNegate when you get the chance. Obviously, MagNegate is far more useful, so it's a good idea to load up on them...if you plan to keep a white innate character around.