Yeah the only times I've managed to beat a group of three or four co opers have been when I got lucky with a force miracle, scared the bejezus out of them with status effects so they roll off an edge themselves, trick them into running into some enemies and taking them apart while they dodge attacks, and only once or twice have I managed to take on four people at once.
It really does feel bad getting summoned into a world in Farron or Londo and then recognizing the name as a group of gamers who already destroyed you earlier, have already cleared the area, and already know some of your special tricks. If it werent for all the ledges to jump off of and knock people off, I wouldn't bother with even trying against thanks.
I spawned a lot on top of that tower above the sleeping giants doing Aldrich.
When I saw 4 people waiting for me I returned to my world, I don't even want to try anymore.
Thankfully I was summoned as a blue sentinel yesterday in the exact same place (oh the irony!) and managed to kill two invaders on my own, as the host decided to sit down and watch the spectacle instead. (like I'm the blue guy who is here to help )
That was fun and I'm so sad these fights are so rare