Have run this mission many times. High DPS covers a multitude of sins, plus enc runes can make a big difference as well.
Strategy 1, you want to get to 2nd boss, as soon as possible so that you can use game mechanic to deal with the adds. I am thinking that burning 1st boss makes the most sense, although saving burns for the third boss would be helpful if the icy witch shows up.
First boss emote: Restless Tijoely roars and turns to face playername.
When you get this emote, it puts a rubberband effect on you that draws you to the boss (you can see the same effect pulling mammoths and griffons in EW). I tried hiding behind a pillar, but I think the best solution is to just run to the boss instead. Doesn't seem to hurt that much.
2nd boss emote: Icebound Bledrek begins making gagging sounds while glaring at playername.
Red ring will spawn on targeted player. Red ring location is static, so entire group needs to run out of red ring. In addition, the clicky that you need spawns in center of red ring, so run out, wait for ring to disappear, and then run back and grab it. Clicky looks like sawdust in scorched woods, but dark brown instead. Use clicky to put green ring away from group and fleshy will ignore group run to it, and disappear.
3rd boss: Nothing special about third boss to worry about, except some times an icy witch appears, and puts a massive green ring that does massive damage. Can easily wipe group, but enc runes plus lots of healing can get group through it. Burning through this can also help, but if you survive the witch, not much need to kill quickly.