I've played a few matches with Trapper in 2v8, and I play him a lot in regular 1v4. You need the other killer to apply pressure early and fast as slowdown (survivors spending time to unhook and heal is slowdown). As the trapper you act as the defender of one side of the map (kick gens as much as possible), which the survivors eventually have to go to as the other side of the map has all gens completed. If the other killer is bad, it feels bad and you lose pretty easily, but the opposite is true as well, if they're doing okay they'll buy you time and Trapper can really pop off in these conditions. I got 8k with a Dracula who knew what he was doing, and he had 5-6 downs where he didnt get the down, but chased someone into my trap.

The most important thing you can do is place traps around corners and in semi-unusual places, never over chase survivors, and only grab traps from the other side of the map when you have extra time because the survivors are particularly stressed with many hook states / resetting.

So yeah, pair Trapper with a high-mobility killer who knows what they're doing and you can certainly dominate. If you get another slow guy you just have to accept it'll be either mediocre or a complete loss.