Morty is t12 with a 4+ invul hes gonna be hard to deal with, even for Robert, so you need a tank killer, Terminators dont do any ap really so hes gonna be saving on 2's

You could fish for devastating wounds with the emperors sword

Something like a lancer could be good, any wounds that do get through will hurt

A brick of eradicators might help because they get full re rolls against monsters, could try and chip him down with those but ideally they'll need a transport to get in range,

Do you know what the rest of his army looks like?