Gas arrows and flaming tar is the way to go. Throw in a trebuchet with explosives that’s been sighted in to either the door, or the entrance to the raid camp and you’re in good shape.(Raid camp always seems to spawn in the same general area for us) We held back a level 10 purge with 3 players and 7 thralls. All slinging some form of fire or gas. If the raiders make it to the door the tar finishes them off. We’ve never lost a thrall, and only lost the gate once, but they never even got close to the coffer.