No seals ? Huh - that's impressive ! I tried it once - but couldn't manage it. Last time I only oused one or two seals - those which weakened his powers of regeneration and something else, I forgot which one it was.
What was your party combination Drithius? For no seals, that sounds like it was a suitably long arduous fight. There's a few more of these drawn out battles through the game.
It was myself standing around much of the fight doing nothing as a Geode, Fayris plucking away with incendiary arrows (sometimes +3 elven arrows), Cano laying down three traps followed by a star scatter (ad nauseam), and Forgrimm spamming taunt. Early on in the fight, several diamond whetstones helped (guaranteed 3 damage), but I quickly ran out of them.
Very long and very tedious But definitely gratifying.
…oh, late in the fight, I also put my geode to work with Treat Wounds as short term regeneration for Forgrimm to conserve Oneberry juice.