Feast of Grimnir Secret Boss Guide

So you want to know where the trolls are hiding do you? Well, I found those vomit filled sacs and I'm here to show you where!

The mission starts with a brief section where you are running alongside a cliff face under the open sky, nothing special to do here, all the good stuff starts once you get inside the mountain and the first trolls spawns. Kill that troll and some double doors will open up for you. 2 barrels will spawn after you encounter the very first bile troll, kill the troll and once the doors open there will be 2 barrels before a drop ledge. After the drop ledge another barrel will spawn in the room below.

You only need 1 barrel to do 2 of the hidden trolls, but you need all 3 explosive barrels to do all three secret bosses.

In this room BEFORE you pull the lever, move the barrels into a corner where they will be safe. Once they are safe, pull the lever and kill the 1-2 trolls that spawn.

Once the trolls are dead send some ahead to clear the way for barrel carriers, NEVER let the barrel carrier go first. Carry your barrels up the ladders and down into the main room.

Once you get to the main room the order of the other rooms are
Waterwheel room this requires 2 barrels.
Fat pipe Door that goes to the open air waterfall this requires 1 barrel.
Cog room or Assassin room, It has a the nicest door 2 barrels can be FOUND inside.

Once you START the waterwheel room there is a lever you pull, that lever destroys a nearby wall marked with ^ ^ stacked on top of each other on a blue flag. ALL similar walls throughout the map with that blue flag are now opened. This is important because in the Fat pipe room there is a free barrel hiding behind an identical wall. And a second barrel is hiding in the Cog room behind a third wall.

I won't lie, the waterwheel room is the most difficult. Don't be scared to throw the barrel to get it away from you. The enemy is hunting YOU, not the barrels. If you throw the barrel and recover it later it will take a few extra minutes, but you'll still be able to get the boss.

Waterwheel room summary.
Go behind and under the drawbridge, cut the chain, get on top of the drawbridge, throw barrel 1. Second half of the drawbridge will lower and you can run across it for a resupply and a barrel wall. Blow that for the first boss. After you kill that boss exit the waterwheel room with all haste as sticking around will just cause enemies to spawn infinitely. Once you exit this room you'll get a small wave of slaves. Kill them before retrieving the barrel.

Fat pipe door
You need 1 barrel from the main room, this will be your last barrel. I generally clear the lowground and the bridge then double back since no more enemies will spawn after you clear the bridge. In theory you can take the middle road and grab a second barrel on the way up but that never works for me. ymmv.

SUMMARY. Run all the way up to the bridge with 2 stormvermin on it, kill everything you find. A wave of slaves is guaranteed to spawn so drop the barrel somewhere safe and just kill things. Kill the stormvermin, the catcher-rats, the gunners, the assassins, ALL of them. Then once you reach the lever, DO NOT pull it. Go back and find the barrel you brought and the barrel that was hidden behind a wall. the double ^ ^ blue flag will mark the broken walls. No new enemies spawn until you pull the lever. So leave it alone. Grab the 2 barrels, use them to break into the boss room via the glowing golden barrel placement indicator walls and you're good. On the way out, GET TO THE LOWGROUND ASAP. Everyone thinks the highground is cool, and it's a tiny bit faster but once you kill the boss here the goal is to leave, not fight. So run and don't get picked off.

Now that you're in the main room head into the last door. The Cog room.

This room is weird, it'll spawn 0-15 slaves and about 10-12 specials almost ALL will be assassins or fatties. Kill the specials and then explore. No enemies will spawn until you pull the lever adjacent to the cog. Ignore that one. The lever you want to pull is next to the waterwheel. Pull that so the lift rises and you'll get a barrel from it. The other barrel is kinda under the bridge in an alcove. If you explore it's easy enough to find. The troll here is hidden near the very top of the waterfall. You have to do a little parkour to get back there. Nothing special, normal jumps will get you there.

Tada, you've now found the three secret trolls. Good hunting...
FOR SIGMAR!

First off, you need to familiarize yourself with explosive barrels, learn to hug those fragile bomb eggs and throw them without blowing up. Which also means you CANNOT take even a single hit. If an ally hits you or even a weak skaven slave slaps your butt the barrel will explode. Don't let that happen.

Don't be a teamfcker who gets yourself blown up. Remember you can push while carrying the barrel. It aint much but it has saved multiple runs. Push and then throw the barrel somewhere safe.

NEVER let the barrel carrier go through a door first. Have someone lead the squad by about 10 meters. Far enough that even if you rush a warpfire thrower and get pushed back the barrel carrier will NOT be hit.

Also, make sure whomever is in front has a gun. If a ratling gun spawns you are NOT going to be able to stab it before it fires a few hundred rounds. If a single round hits the barrel you missed out on a boss.

My personal thoughts are that this event is too strict with the barrels. There should be 1-2 extra barrels and they should be in the main room just in case people spawn in late. Right now they are too easy to miss, and it requires more coordination than feels fair or reasonable.

Suggested team comp.
Grail knight for DPS
Necromancer for barrel carrying / protection
Bounty Hunter (dps and special sniping)
Engineer for free bombs and always being able to shoot.
Shade if you can survive and backstab, very hard role but very good.
Replace any of these with a tank if you are having difficulty staying alive. Otherwise you are kinda squishy.

Special sniping is critical to keeping the barrels alive. If you don't have the above classes, here are a few alternatives.

Runner ups
SOTT, deepwood staff for infinite ammo and lifting enemies if they get too close to a barrel carrier.
Witch Hunter, tag things and shoot them. A great team dps boost.
Handmaiden, movement and stamina regen aura for carrying barrels and cogs.