Primarch Roboute Guilliman (Warhammer 40,000)
VS
Easdeath (Akame Ga Kill)
Reason: Two revered, high ranking members of two of the most brutal empires in fiction duking it out. Esdeath to prove that she's the strongest, and Guilliman because... well, Esdeath probably did something heretical. As to who would win... well, I mean it's kind of obvious. I don't actually know anything about Primarch Roboute Guilliman other than he was revived by an Eldar woman, but given the sheer nature of 40k, combined with the fact that Esdeath's a bitch, it's not even a competition. He would probably stomp in her skull in three fell swoops while his Ultramarines mop up that circus Esdeath calls an army.
There are none who can oppose the Imperium of Man. FOR THE EMPEROR!!!