I used to think this boss was completely and utterly impossible unless you had insanely high DPS in your group and could just burn him fast before the first atronach enrages. I took a lot of practice, and trial and error with different strategies, but now I actually consider Ibomez The Flesh Sculptor to be one of the easiest bosses in this dungeon.
I am a magicka templar and have beaten this boss both as healer and DPS. I can do around 17k DPS, so definitely not super high, but this was not a problem at all. I think the nerf a few patches ago probably helped, but mostly it was just a lot of practice, knowing the mechanics, and good communication and cooperation between group members.
Short explanation of the mechanics:
* Inmates will come from 3 doors: north, east and south of the room. Green grenades will spawn randomly on the ground. You need to synergize with the grenades (press X on PC) and throw them on the inmates. If enough inmates make it into the pool at the center, the boss will use them to summon a flesh atronach.
* The flesh atronach enrages 60 seconds after he spawns. When he enrages, he one-shots everyone including the tank, and it's a wipe. If you throw grenades on the Flesh Atronach, he gets covered in green poison which makes him significantly easier to kill. This debuff must be refreshed every 10-15 seconds by throwing another grenade.
* Every once in a while, the boss will choose a random party member and "tenderize" them. The targeted player lies helplessly on the ground and will surely die unless the boss is bashed right before he does the final attack. The usual interrupt cue will appear when it is time to bash him; do not try to bash before that, since you will just waste stamina.
* The boss also does a heavy attack which can one-shot the squishier party members. These attacks can be bashed or blocked. He will also do a cone AoE poison attack which can also one-shot you. It can be dodged, blocked or absorbed with a shield (e.g. Harness Magicka). If blocked or absorbed, you will still get a heavy DoT on you for several seconds.
Here is the strategy that I found the most useful with my groups:
* Needless to say, everyone must be using TS and a mic because communication between party members is key!
* The healer is dedicated to throwing grenades on the inmates and atronachs. There is not much healing needed in this fight, especially with a good tank. Breath of Life is the preferred healing spell since it has a very long range and does not need to be targeted, so the healer doesn't have to stop throwing grenades. If someone dies, the healer (preferably a templar with Kagrenac's hope) needs to resurrect him ASAP since every second of DPS is crucial in this fight.
* One DPS, preferably the one who can move faster and do ranged DPS, also throws grenades. Whenever possible, they should add DPS on the boss and atronachs, which is why they should ideally be ranged. However, throwing grenades should be first priority.
* The second DPS, preferable the one with the higher DPS and with good sustain, and ideally with a good AoE, is dedicated to damaging the boss and atronachs. Ideally he should AoE the boss and atronachs together until the atronach dies, and then focus on the boss. He also watches the debuff on the atronachs and asks on TS to throw grenades on them when needed. Note that if the atronach is at less than 10-20% (depending on your DPS) and is not about to enrage you can just finish him off even if he's not poisoned; better use those grenades on inmates.
* The tank keeps the boss and atronachs together around the center so that the second DPS can AoE them. If possible, he should also take some pressure off the grenade throwers by throwing grenades on the atronachs if there's a grenade near the center. When the boss is tenderizing someone, it's the tank's job to bash the boss. If the tank is being tenderized, the second DPS (the one attacking the boss and atronachs) should bash the boss.
* The two people who are throwing grenades should split the room: one takes the north side, the other takes the south side. The east side is shared between them. If inmates come through the door on your side, it is your responsibility to throw a grenade on them. Stay on your side of the room and only pick up grenades on your side since two people trying to pick up the same grenade is just a waste of time. Be sure to communicate over TS, for example: "I'm being tenderized, please take care of the inmates on my side" or "it's okay, I got the inmates on the east door, get the atronach".
* No matter how good you are with the grenades, unless the RNGesus is on your side, some inmates will inevitably reach the center. When an atronach spawns, the first priority is to take him down. He needs to be covered in green poison at all times. The second DPS should be able to handle it, but if there are two atronachs out (which might happen if you were very unlucky with the grenades) the first DPS should stop throwing grenades and help take them down quickly before they enrage. Obviously, kill the one with the lower HP first since he will also enrage first.
* When the boss is around 10-20%, with the optimal value determined by your group's total DPS, stop throwing grenades and just execute the boss. Congratulations, you did it!