The trick we found to Primordius is just to constantly kite him. Like WAAAY more than what you'd think. It really helps because it gives DPS a chance to kill the slimes as they're reaching him, which slows down his evolutions. Once we figured that out, it was a 1-shot and it's a 1-shot every time.
That's what we do, what we've always done. For some reason, on Thursday, it just Didn't Work. My theory, as I said, is lack of DPS, but it was a surprise, given how easy we've found the fight in the past.
Durumu is a POS because of his Life Drain, really. The rest of the fight is cake - the maze is really easy, and you just need to be able to see a bit and you're good.
I think the reasons we had difficulty with it in the past was that it's a very reaction-based fight; there's quite a few mechanics that you need to get out from / react correctly to immediately. It's perhaps the first fight in ToT where one person out of the entire raid can screw it up for everyone; in all previous ones I can think of, there's quite a few roles where you can put the couple of members who tend to Screw Things Up to minimise the damage they can inflict; on Durumu, though, there's nowhere for them to hide; if they get beamed, they need to handle it right. If they get knocked off the edge or screw up the maze, your DPS is severely damaged and, as I mentioned before, DPS timers on the fight are tight.
We 3-heal Lei Shen now, and we downed him last week. Our average DPS was around 115k, which really wasn't bad, and it's really a pretty easy fight once you control everything. It's simply not a fight where you can just go balls to the walls - you have to control when you push him into the intermissions so it's safe (especially intermission 2 where if you push him while lightning orbs are up you're essentially dead) and destroying the right conducts (we decided to get rid of chain lightning first as it lets us be grouped for intermission 2, which helps immensely).
We've had orbs on the intermission and it hasn't proved *too* bad, actually. Possibly due to the fact that we're extremely careful with positioning in the last stages of phase 2, meaning that when the intermission starts only the tanks need to move. It's just a case of getting control on P3 and then we'll get the kill.
As you said, I think ToT is badly tuned for 10man. Horridon is too difficult until you figure out the trick (which is absolutely stupid - tank Horridon at the far end of the arena, opposite the door you're doing, and you essentially remove the charge),
We never really found *that* to be the problem on Horridon; for us the difficulty lay with the second door (venom bolt volley out of control) and the third (untankable adds requiring good raid discipline to keep them grouped. Charge didn't really figure as a stumbling block for us.
Tortos is a little bit too tight,
My only real complaint for Tortos is the fact that the kicker needs to take into account time-to-impact for the shells - that caused us quite a few early wipes. I'd be happier if there was a way to train that skill independently of the raid environment.
It's just a weird raid and the balancing is off. The last few bosses being so easy (once you're past Durumu it's smooth sailing up to Lei Shen - they're all RIDICULOUSLY easy) is just mind-boggling,
One thing I *like* about the raid design is that there's a very clear story progression to it; the front lines are the Zandalari defending themselves, then you detour into the caverns and have to deal with the indigenous life, then you enter the regions where dark experiments are taking place, before finally finishing up in the Mogu palace.
Unfortunately, that means they couldn't really reorder the bosses into a better-paced order, given each one has plot reasons for existing where it does!
(That said, one thing I notice has sort of gone out of the window - or if you like, it's still there, but done to a ludicrous extreme: I'm sure I recall blues talking about the idea of 'reward bosses', fairly easy challenges deliberately placed after tough fights to allow a slightly less stressful section to reward the players for progressing past the challenging bit. Either they've stopped doing that, or they think that three challenging bosses followed by a reward of two easy ones, followed by one challenging boss then a reward of four easy ones is a good approach!)
and it's obvious that they decided at some point that having so much trash between bosses was stupid, so there's very little packs between the bosses after Durumu. The gauntlet leading to Primordius isn't too bad, the pull before Dark Animus is harder than the fight itself (lol) but there's a SINGLE pull... Then you compare that to the absolutely absurd amount of mobs between Jin'Rokh and Horridon, then Horridon and Council, Tortos and Mag, Mag and Ji'Kun, etc. It's just... dumb.
At least Council -> Tortos isn't that bad!
One week we got to Megaera and then the serves crapped out on us - no-one could get off a loading screen, meaning nine of us were in the raid and no-one could join or leave us.. To kill time while we waited to see if the servers woke up, we cleared the entirety of Megaera's room. NEVER AGAIN.