@Big_Boss shotguns inflict significantly lower damage than a rifle round does. The pellets/slugs also move much more slowly, and since these things are already dodging high speed rounds at least some of the time...
What he needs to do is switch to smaller battle rifle, like the SOCOM 16. Same ammo as that SCAR, but much shorter and easier to use in CQC. It also mounts a bayonet to make CQC more lethal and gives him an option other than "grab the incoming sword blade" when pressed. Alternatively he could swap to an assault rifle since they do almost as much damage as a battke rigle and are much better suited to massed enemies in narrow confines like a dungeon's corridors.
Go Belt-Fed or Go Home! I dig it. Hard to use unless he goes prone though. He'd be better off leading the assault with a couple of flashbangs or frag grenades, and then following up with assault rifle fire from behind the Tank's shield.
I'm trying to figure out why he didn't immediately pull out an RPG or AT4 when the hobbo mage turned up lookin' all demon-y and shit.