How to counter third party

for example, i died a lot of times third partied this season even though i was killing players really fast, like it took 10 seconds to kill and finish fight and another guy just comes at me, and then another, and so on. Oni masks, would be perfect if you could break portal by shooting in it. Shockwaves are annoying, but counterable. What could i possibly do, to counter third-parties (except positioning.)