S.K. Ren
Prince
I figured I'd chime in since I just beat the bastard. Playing as the Svartlfar on a Monarch Tiny map, I took Acheron down with a 64 Strength Yvain (Combat V, Drill I, 12 Nature Mana) after softening him up with some Nyxkins. Afterward I used the rest of my Nyxkin to soften the Sons and Killed them all with assassins.