Nearly everything DS does could qualify, but the entire fight with Fallen Uriel is unquestionably the greatest so far.
Dark Schneider saving Arshes Nei by ripping out his own heart and letting the blood fall on her cursed fingernail, sparing her the fate of being turned into a toad. Just before, he became so enraged at the revelation that she was cursed that he single-handedly nullified both hers and his own Halloween spells. It doubles as Heartwarming as well since it looked as if he truly killed himself to free her, but...
How does DS beat a flame elemental? He fires off an even bigger flame spell on top of Ifreet's own spell.
Ifreet: "How can you summon a flame hotter than the sun!? HOW!?"
D.S.: "Idiot! I added my power to your Hellion spell!"