While this isn't the topic of this thread, but how about you start by naming your tanks and healers. Which class are they, how many of them you use. What do they do (ok, more like how, rather then what? heal).
For example if the tank is a dk its one thing, if its druid its second, if its monk its third etc etc etc. Also some classes have it easier with their active mit while other may need more of planning. Not to mention gear plays big role too. For example if tank may is not geared enough he might need to gear towards stamina more, he will lose of stats he actually wants. You get the rift.
Also as for the healers, one tend to focus more on direct heals, other may be hoters and their heals come not when you need them but some time later. you might also have or not have any absorbs etc etc. Until you provide more info no one will know what the problem is.
But to be precisely on topic, if all raiders are honest enough to tell on their fails right away, then 3 tries are enough to start seeing patterns.
Of course to see them you need to have info on whats going on.
For example if horidon tank is loosing agro (lets think hes undergeared) he should have omen and know that here I was on first door, tanking like a boss. Since it was the start of combat I had that lock/mage/enh shaman almost caught up on my threat. First doors adds appear but that lock was still on my heels. So you start looking into what was lock doing. Was he actively single target nuking the boss or was he just keeping his one or two dots on boss. If its the later then your tank needs to do some research about his rotations (maybe hes missing something like not using mangle as bear).
The very same thing with healers, lets say boss tank died. You ask what was the problem. he says no heals. You check the logs and see he received no heals for lets say 10 seconds. You can clearly ask the question, wtf were healers doing.
So all in all, more info please.
Our guild hit the Horridon wall early on and wiped about 20 times on 2 different nights, then our Raid fell apart and we haven't raided since. This was all around April-May. There shouldn't be a boss like that in the first 4-5 IMO. Our issue was Healers and one shit DPS, basically we had a group of 2 good tanks 3 good dps and 1 good healer and had to deal with carrying the rest of the raid.
What of the other players who would be happy to perform that roll and excel at it, but can't because they can't find a group to join? He's wasting their time too.
Is he solo tanking Horridon or are they switching at each gate?
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I would almost bet the DK isn't using Death Strike enough to build up the absorb which as I understand is the DK's primary active mitigation (you'll pardon my ignorance, my DK was abandoned at 90 and was DPS, although I tanked a normal Jade Serpent run at 90 just to see it): Death Strike to build up a shield, Death Strike after the hit (Triple Puncture in this case) to heal back up.
Took us ~200 times to get H-DA, but got to 14% on H-Lei Shen after 25 wipes, it all depends on what your group is doing wrong and how to fix it.