Oh, wow.
I opted not to train too much before taking on the Elite Four. I was impatient. I actually beat them easily, but N and Ghetsis, that's was a different story.
I still refused to train even when I had the chance to leave the castle. Stubborn little me.
My strategy mostly included something along the lines of attacking, healing, attacking, more healing, sacrificing to revive much more often than I should have, damn you Hydreigon can't you just die already? STOP SWEEPING MY TEAM.
And of course: HOW DARE YOU USE THAT FULL RESTORE.