That's not how the one shot works. The one shot only happens if you have more than 16 people in the same operations frame. You can actually have two separate Operations groups tag-team him without a problem; however, only the group that engaged him gets the loot (and no, I've never done it that way). That being said, the OP isn't getting one shot - the Imps are just causing wipes by taunting and turning the boss to raid, etc.
Are you sure thats what happens? Im pretty sure the issue is when he has more than 16 people on his threat table it increases his AOE pulse by 200%. The whole reason its even there is to prevent your raid from receiving outside help.