Round 1: Given that Grigori outside of Bitterblack Isle is stated to be, like, Planet level, and the actual page for White fatty is, like, Moon level, Grigori should be stronger I guess. However, it was largely agreed upon that White Fatty is Planet level in
This thread, but is only listed as moon level until the calcs for White Fatty are "sorted" through. So, in that respect, it would be even, but without that, White Fatty uses Dragon Element, which deals damage to a monster based on intellect, and Grigori seems to be highly intelligent. Still, Grigori can use fire, which White Fatty is weak to. So if White Fatalis is accepted as planetary, he wins with a
lot of trouble. If not, I say Grigori wins, though with mid-difficulty.
Round 2: White Fatalis is actually a good deal faster, being relativistc with FTL attacks while Grigori is Sub-rel. And everything he had in Round 1 is largely applicable to this round as well. If moon level White Fatty, I would give it to him with high difficulty, but planet level White Fatalis takes with mid-difficulty
Overall: Round 1 goes to Grigori, only if White Fatalis is kept at moon level, Round 2 goes to White Fatalis on account of being faster and having attacks so fast that Grigori could simply fail to dodge them/react to them.