The Magnum Opus? Yhea, we're going to need that, it'll buff your stats up to a point where you're a force to be reckoned with...Assuming you're comfortable with the Hammercannon, it might outdo the Pipsqueak as a good weapon...

I have a risky idea involving Nightmare. Nightmare is invulnerable to every attack except form the Star Rod, and I don't think Nightmare appreciates Multidimensional collapse...In other words, all we need is the Star Rod, and we can force Nightmare to our side, and we have someone that Reverankh can't beat. At least, that we expect he can't beat. It's risky, though, and we would be unleashing Nightmare...Who we may have to defeat again.

The Wither Skeleton army will likely leave whatever undead forces Reverankh has be, as the forces of Witherkind attack anything that isn't undead, which includes machines but not zombies or the like. I believe, however, that the forces I have gathered is sufficient enough to keep the majority of the army at bay, if their high levels and their conversion of skeletons to their side keeps up.

...Pokemon! We'll need all the help we can get! Whip out whatever Pokemon you have, quickly!