My Symurgh has bite, blow and rush all above 100, but it appears to only be doing around 45 damage while fighting Tundra monsters? Even with Paws boost, it's only doing around 60-80 damage?

Or is the stats just for critical hits...?

EDIT: figured it was slowly decreasing the damage you do to up the difficulty.

The deeper is the level, the higher stats the monsters have. At a certain point all of them have the max 50% damage reduction due to high stats.

Are their critical chances buffed and ours nerfed? I seem to be unable to land any critical hits at all.

All basic battle parameters depend on characteristics. So the deeper the level, the more constitution and intelligence monsters have, and the higher their anti-crit is.