Well if your aim is good, like you said, try using 3 Adv. Debilitator Mk I and 3 Magma Hammers Mk V. Group them into two separate groups and you're good to go. The trick with the Executioner is to stay on the opponent's ass all the time. So slide around him and behind him and use your superior aim to slowly eat his hull out. Or you can use full Magma Hammers, 3 Mk IVs and 3 Mk Vs.
Forgot to add, pick your shots, don't spray them around because the powercore is small. And don't use Vengeances, they make you look like you're pissing on your foe.:lol: