If you're running into a bunch of hard to kill
MCs/GMCs its pricey point wise, but a seige claw Leviathan will make short work of them, Easy, And it fits the Fists
Also a Typhon is very -very- anti tau and eldar (But this is an -extreme- choice)
Medusas with Phosphex shells are awesome marine (And hordes in general) killers, and can do a number on things like necrons with the basic shells.