I'm sure you're all sick of people asking this, but my raid group has been bashing our heads against Horridon since late March and we literally have not seen the fourth door yet.
Unfortunately, I had some pc problems recently and didn't end up logging most of our runs, but I do have logs from our most recent raid night to look at. I've never posted here on mmo-champ before, so I can't post a link, but we're Earendil Riders on US-Moonrunner and the six attempts from that night are (I believe) the only report currently up there.
In terms of strategy--we don't do anything crazy. We tried three-healing on our first two attempts that night, then switched to two-healing when that wasn't working too well for us. Tank swaps after each door, and we would say to always kill the second major add (always doing the one that spawns on the left, so there's no confusion about which to kill first) before swapping to the dinomancer, even if the second add is still fairly healthy. I realize this isn't ideal, since it gives the dinomancer time to heal Horridon and lets the little adds build up, but to be honest I'm more interested in getting to the Horridon phase than I am in getting to it with him sufficiently damaged. When the sand traps/living poisons/frost orbs start to reach dangerous levels, we start moving the raid towards the next door to keep people from tunnel visioning their way to death.
In terms of issues I'm already aware of--first of all, I realize my healing is pretty suboptimal--in particular, I was appalled at how low my lifebloom uptime is, and I sometimes fail to use my cooldowns as often as I should (Ironbark in particular, I forget it extremely often). Also, the second and third doors can get pretty hairy when the debuffs stack up, since when we 2-heal our paladin tank and myself are our only two capable of dispelling the diseases, with our moonkin able to help on the poisons as well. Additionally, we haven't been using Heroism consistently, although we're starting to just go ahead and use it on the third door (again, I'd rather save it for Jalak, but if we aren't even getting through the doors there's no reason to go without it).
I'm really interested in any feedback we can get, to be honest. This boss is driving me up a wall. In particular, I feel like our DPS is far below where it should be--everyone in the raid is over 495 item level, most of us are over 500, and yet we struggle to kill the major adds in a reasonable timeframe. Unfortunately, I don't know enough about most DPS classes to read the logs and see what they might be able to change to improve. Any advice for myself, the other healer, or the tanks would be greatly appreciated as well. I am considering the strategy I've seen of just sending a healer and the Horridon tank to stand on the opposite side of the room so there's less to worry about, but (A) I'd prefer not to resort to trying to break the fight, and (B) I'm concerned that one person might struggle to handle all the raid healing by themself, particularly if the various DoTs start to stack up because the adds aren't dying fast enough.