When fighting VC or the captain the healer jumps onto the cannon beside the ship where no monsters can reach them. It gets even worse though. The puller positionally aggros the monsters and pulls them to the ramp leading onto the ship but doesn't hit them. Once the puller gets on the ramp, the healer on the cannon heals him - this aggros the monsters on the healer but they can't get to the healer. The monsters then go into the perma-evade state attacking no one and just standing there. Every once and a while a monster will pop out of evade state (or the healer can move around to make them do this) and the group can kill it.
This exploit allows a group to effectively fight the captain and the van cleef encounter one monster at a time, completely trivializing them. The group gets to focus on one monster at a time while the rest are stuck in evade mode because they can't reach the healer.