Garou scales to Sage Centipede, who's 2.46 petatons. Akame scales to Esdeath, who's 1.58 petatons. Garou has a ~1.56x AP advantage, not that extreme.
Garou seems to have all of Akame's skill feats but better (predicting attacks, identifying weak spots, attack reflection, afterimage creation), not to mention that he's constantly getting stronger/faster/tougher as the fight goes on. There's also the possibility of Garou stealing Akame's fighting style and catching her off-guard with it, considering that his copying is potent enough to copy nonhuman fighting styles like Watchdog Man's.
The only trouble Akame can give Garou is Muramase because of its poison/curse, but that's assuming she can even cut him.