So you need to kill an invader with a hand cannon. I had exactly one successful strategy to make this bearable. Play in a party with guys who will call out invaders for you. Boop them with your tractor cannon to shut down that overshield/super cheese, double tap with the hand cannon.
I hated this step too. Guys who have also done it, comment your strat. Maybe we can help out some blueberries.