Horridon has a long learning curve. I suggest you use your CDs early and often so your team can get as much practice in as possible on each try. One ability to watch out for is dire call -- heavy raid-wide damage on 1 min cd. Sometimes it will occur during transitions between doors, where it will not pose much of a challenge. But when it happens concurrently with various adds swarming your raid, it will almost certainly require major cds to survive. The first really dangerous dire call occurs around 3 minutes into the fight on door n2. I suggest you have tide ready for that one. The nastiest dire call comes towards the end of the fight (around the 8 minute mark I think), when Jalak is up. You raid should be stacked at that point, so this is the perfect time for spirit link + ascendance -- pop them before dire call hits! Other dire calls aren't that critical (providing adds are dpsed down fast and in a correct order) but you should work out a cd rotation with your co-healer(s) anyhow; even if doesn't save lives, it will save mana.
It depends alot on whom you heal with and how long you think the fight is going to take (if it's disc priest+resto sham it's the same setup as we use). As you learn the fight just pop them as often as you can. But when you know all the doors etc I pop tide at door 2-3 and that makes it available again in the end. Didn't use SLink until the end stacking and the same with ascendance, I should probably try and use them some more but there just isn't that much to do or a need for them. People that die usually die to stupid unhealable things anyway like standing in poison/sandtraps/frostballs/lightningtotems etc.
Overall not much happens in the fight once you learn to interrupt the various door and controll them. Dmg wise it's lots of minor healing, dispelling, followed by giant spike, minor healing, spike as you progress from door to door. It's easier I think, or we found out, that if you stack-spread up at spots on the various door. That way you can rain on them.
I really hate the tuning on the fight for 10man heroic. Bring the player not the class my ass. Only having one ressurection is a bit fail and harsh to since you kinda need it in the end so if you use it before you kinda want to wipe; atleast after you have learned to various doors and want to have it until the end. All in all I think it took us around 35-40 pulls to kill it.
(edit) I forgot; You can push your pink dino away with Searing Totem and Purge/Flame Shock. Just put the totem down; purge the dino (it doesnt have a cd) and the totem should take care of him for the durration of the totem. Watch the totem a bit tho cause sometimes the mobs at door 4, you shouldnt get pink dino before that, sometimes like to destroy the totem when they spawn.