Easy mode cheesy: Gjally spam, done in seconds
Easy mode legit: Ice Breaker. You'll never run out of special ammo and even without the buff you can break the detain using IB. Shotguns do it in time as well (full clip of Found Verdict does the trick) but you run a far bigger risk of not having enough ammo inside the bubble and dying. If the goblin is in sight and you kill it IB breaks detain in like a shot Which lets you easily break your friends out. A good scout rifle for boss damage peppering and heavy whenever you get it. And patience. Lots of patience
Make sure at least one of your friends has a good shotgun in hand for Major duty. Invective works great for this fight since it's so long, ammo can be the killer.
It's a long and tedious fight but once you get the hang of it you'll see it's not hard at all. Just annoying
