My Balance had to go through that quest too. It does take a while (and a little bit of luck IMO) to defeat him. I have a secondary deck for fighting balance bosses and mobs and the key spells in it are: Chimera (You need to be level 60 to get this spell), Hydra, Spectral Blast, Satyr, Polymorph Treant and Power Nova (for adding a weakness to them).
I hope this helps you with defeating Grignr and good luck getting the Judgement Pet
-- Timothy EarthHeart lvl 75 and Timothy FrostHeart lvl 63