IMO you want to compare the best Kagero vs the best Gaius. Personally I think the best Kagero is Darting Blow ('cause if she drops her defences she's never taking a hit) while Gaius has the benefit of 42 HP so he can afford to use Life and Death instead. Even with Life and Death factored in you'd need 60 Atk on the physical side and 57 Atk on the magical side to kill Life and Death neutral Gaius in one hit. A feat that is completely impossible through Atk alone. To get to those kinds of levels you need an A skill, a special skill, some buffs, or a Brave weapon to be helping you out.
Offensively Life and Death Gaius boasts 39 Atk and 41 Spd with a Poison Dagger+, while Darting Blow Kagero has 40 Atk and 38 Spd. I think you'd agree that 39 Atk would be sufficient to kill practically any infantry unit when doubling, so Gaius' slight Spd advantage when attacking, and large Spd advantage when defending give him the upper hand.
Like I said, it requires very heavy investment to make Gaius worth it, you need to pass him Poison Dagger+ from a 5* Kagero, and Life and Death 3 from a 5* Hana/Jaffar, but he does have the base stat skeleton to be the best offensive dagger user.