This one is currently the toughest of the CoV Hero missions imo. The Crusaders are tough for high end group gear, and even raid geared group atm and it requires quite a bit of healing.
I will caveat that I often box most of it and so the lack of warm bodies to focus on negating the powerups (power grows in <NE,NW,SE,SW> corner emote) makes it a little tougher, but I have been able to get the Savior achieve for not letting them power up anyway a couple times. Beyond that, the real problem is the amount of single round damage you can take - the Crusaders can have ~80-150K DoT tic (and DD) and the adds you get can do 200K DD in a single round. This with the debuffs and other nonsense, it is bad news even with a considerable amount of healing. There's multiple DB forum posts on it, luckily it sounds like it will be tuned soon.
Until then! A note on the (current) effects:
-Jorteva's is nasty as hell 100-150K DD then same or close in DoT. This definitely dictates what order you want to tackle them in.
-Charayan is the minor melee slow + DoT as noted in the walkthrough, but DoT is also like 100K. It ALSO has a mana drain.
-Susarrak is the bigger melee debuff, so just impacts your burns and if you are a melee heavy group this is the one to figure out where in the rotation you want to engage.
-Grendish gives a massive AOE movement slow (90%?) with a 150-175K DD, and mana drain since it's not currently listed.
Knowing the effects, you have to pick your poisons on the order of engaging the Crusaders. You are essentially hedging your bets on how often the remaining Crusaders might reprise the effects of the others, but you are shaving off the high points and making it more predictable. I have done it starting on either side and going clockwise/counterclockwise, died a boatload of times, and so far the easiest success has been this:
Jorteva > Charayan > Susarrak > Grendish
Get the nasty DD + DoT out of the way. It can be hard to heal and nothing sucks more than getting the other 3 down and then dying (repeatedly) to this dragon. Once that is done, you can deal with another semi-nasty DoT - the key importance of tackling these two is that you typically get the adds (stupid chromadracs) on the 3rd & 4th dragons.
Now, what you want to do is pull the dragons as close to their corner, and probably a bit south/east as you can. The 'Cleaning Cube' that wanders by will port any player it touches to almost certain death. You may as well reset if that happens (and you don't have a mage who can CoH them back before they die or you do), because clearing all the mobs from where it takes you is too much work with the run it takes to rejoin your group if it happens. I position the group in the corner, pull the dragon into it, turn it around so that only the tank is near the hallway, and fight with an eye out for the cube. TL;DR avoid cubes at all cost.
You have to kill the add while engaged with a Crusader, so kill them with priority and if the Crusader is close to death you need to drag the add with you to the next one. This is also why you may want to keep Grendish for last - that movement slow and the add summoning can screw you for getting engaged on the next Crusader and reset, which is very frustrating.
Knowing all this, just stay healed and steadily DPS first two Crusaders. Then I basically start burns on the first add that joins (usually 3rd Crusader), then take down 3rd Crusader as quickly as possible. Move to the final crusader, pop any remaining discs/burns, and when the add joins prioritize based on how close to dead the Crusader is.
After that, simple as walking into Yelinak - who looks awesome - and opening the chest.
BE CAREFUL of the cube on the last fight, it keeps being bugged for me and instead of pathing squishes itself into the corner, hence pulling back a bit down the hallway away from the corner (but still hugging the wall, near the ice pillar thing).
Hope this helps, and ideally this post will lose value when they tune this mission appropriately
