It seems like a lot of your issues come from raid damage that can be mitigated, make sure that people arent standing in frozen orbs/venomous effusions/sand traps/lightning novas. Also, the poisons on door 2 can quickly add up to a lot of damage, so make sure that everyone who has a dispell or cleanse is using them, this save healers a lot of hardship and reduces the amount of damage that your raid is taking. I tank this as a protadin for my group and usually have Horridon on door 2 and make sure to use my cleanse whenever it comes off CD. Same thing with the diseases on door 3. Also, make sure you have assigned interrupts for the venom priests on door 2. You can see them in the stands before you even pull Horridon so you can mark them and sort out who is interrupting what before the fight even begins
Also, since you have a DK on your team, have him pop Army of the Dead on the third door, they help to soak up the diseases and make things easier. Remember that the only adds you can control here are the frozen warlords. My team also found it effective to just stack on the add tank here and the add tank would start to kite whenever the circle for frozen orbs appeared. Like you we also got stuck on door 3 for a while before downing it, so we decided to use hero here to make sure that the priority adds died quickly so that we spent as little time on this door as possible.
Overall just make sure your dps are prioritizing the right targets and not just aoeing everything down, and make sure everyones aware of things that they may or may not be standing in. Honestly my team found the 3rd door to be the toughest part of this fight, and when we finally got it down we easily cleared the rest of the fight. Good luck downing him!