It's almost like you guys are slowly leading up to the conclusion I've already made. ;p
If a hidden crasher rocks everyone up by surprise, evaluate the amount of time you have left. Unless you have like 1:40 left on the clock, don't even bother trying to regain the lead, and proceed to run away for the rest of the match and continue taunting when possible. If you really want to, toss an annoying as hell projectile around just to needle him or her. Loop Pit Arrows all over the place, use Snake's Nikita or Grenade Stripping techniques, etc. Keep yourself occupied and just do your best to avoid/annoy the sudden crasher.
When the match ends, one of two things will happen.
The first one is that the crasher will leave, feeling satisfied. This conclusion is, tbh, fine, because you were not outskilled in any way shape or form, AND you had a good enough time being a pain in the ***. Hopefully, you had some good fun, and there's no shame or hard feelings in losing a battle you never even fought. From here, just continue to arrange your parties according to the normal agenda.
The other conclusion is, obviously, the crasher staying in the room. If this happens, go wild and proceed to **** the living **** out of him with your 3v1 strategies, generous 2 minutes of murder time, and most importantly, the evened out scores.