I just "lost" a game which we were 3v0 because it was slaugther mode, no one to kill and my npc buddies kept dying
Of course, this problem should be fixed by release, but meantime:
When you get into a game like that the only way to win is to kill the enemy citadel. It has ugly thorns though, so you want to make sure you don't capture enemy portals so that you can level off the grunts enough to survive the backlash. Don't bother with your major PK nukes, just invest in stats and health and try to get into the enemy base while your allies are still destroying towers so you can win.