Is there anyone outside of Glory, Illyria and The Beast he couldn't beat? Strength enough to casually rip off a vampire's head, can tank Buffy's attacks no problem, armed with a Gatling gun and grenade launcher...
You need either sick levels of soak or hax to beat him. I don't think anyone but the trio i mentioned can cut it. He should at least be above your Hamilton's, Caleb's, etc.. He's a probably in the same "tier" as them physically - being "Above Buffy-level" so above Buffy/Angel/Spike/Faith - but the guns really are a huge advantage.
P.S.
You know those shadow men really did screw over the Slayers. I mentioned Adam had enough raw power to just decapitate a vampire with his bare hands. You would think that level of strength would be handy for a Slayer so she isn't dependent on a weapon to kill her foes.