Given that I reckon I know this certain player, I have had similar problems and feel your pain, mainly with the obliterators, have you tried focussing on the obliterators with your plasma fire, as you are then leaving him to save on 5++ which ismuch better, this is what I generally do against TeQ especially at T5. Or as Mossy says, try and ignore them unless they become a problem
Yeah, the problem is getting the Plasma to them before they pop expensive, good stuff.
Helldrake = Flakk, as much as you can, from as far away as possible, and the fact the helldrake has 360 fire now is also going to be a problem.
Flak is proving very difficult. I only have the 1 Devastator unit, and as anyone who's played a Heldrake before can tell you, 5 Marines isn't even a minor threat to one no matter what weapons they have. Flak missiles are also pretty awful. One third miss, two thirds of that bounce off armour, a third of that is Jinked, and half will get eaten by Daemonic Possession. Very, very difficult to shut down.
With the Deamon prince, I can't be much help as normally I just throw a c'tan or shackle lord at it, generally I would try to focus on grounding it as a S9 hit is nothing to be sniffed at and provided the helldrake isn't a problem, you could send Flakk missiles it's way, but with 4++ and that damned tzeentch deamon prince rule, no matte what it is always going to be a tough takedown. (I have to admit I am annoyed how much he has used a 3++) And most likely now he will end up taking power armour, so try to find something AP3 or better.
TL Storm Bolters (standard gear on Terminators now
) are up to the job of Grounding it, it was just the 3++ re-rollable or T6 that was an obstacle. We've now had words and hammered out that Nurgle Daemon Princes do not have the Mark of Nurgle, and Tzeentch Daemon Princes cannot take Sigils of Corruption.
To deal with the slaanesh marines, I would either try and get some sniper unit (I though you had ratlings? Yes I know they are bad but it's better than nothing) or try and get into combat with 2 characters, so one can deal with/decline the challenge but you can still use the precision hits, however this is obviously much riskier.
Ratlings are cool, but the chances of getting a Precision Shot that then wounds, goes through Power Armour AND Feel No Pain is minimal. Battle Cannons may be the answer, I just need to work on deploying them better.
With cultists, do the Leman russes not take them out? Otherwise I would just try to mass fire power.
Nah, Battle Cannons bounce right off of 4+ cover. Deathwing Terminators could do the job, but that's a 245pt unit dedicated to killing the 160pts or so of Cultists.
Good ideas though, people, keep 'em coming.