How to avoid your tank getting oneshot
Probably one of the most annoying parts of Tirna Scithe, there are at least two different mob types that have a heavy single-target ability, and they also might come in packs of two which means sometimes your tank is in danger of getting hit by two tank busters at the exact same second and just dying.
These are 322557 from 164920 and 463217 from 166276. For the 164920 you know ahead of time where they are: The very first two pulls of the dungeon have two of them each (assuming you go the usual route, using the shortcut through the vines and then cc'ing one mob to skip the following pack).
The 166276s are a bit more annoying, they might or might not show up alone or in pairs depending on what route you end up getting through the maze, and you might get either none or two different pulls with two of them, so you just have to keep an eye out for the specific mobs you get in each maze pull and be ready for it
There are a couple of things you can do to help the situation here: for the 164920s they are not immune to cc, which means you can either stun or sleepwalk one when the other is gonna use the spell so you desync the casts and so your tank only has to take one at the time. You can't do that with 166276 but you can still preemtively td or tg your tank before the hit to help him survive it.
There is a delicate balance to achieve here, if you use td when the tank already has a big defensive up, then you wasted it and might not have it on another set when they have nothing up and might actually need your help. Also, even if they are ready to survive the hits, they will still take heavy damage and the pulls still contain other separate mobs that will be doing their own thing, so quickly sending heals to top the tank is important so they don't die to small follow up damage right after surviving the scary overlap
The best advice here is to have an easy and reliable way to track what cds your tank has active and available, and also the cooldown of mob abilities. This way you can try to predict what is gonna happen on following sets of casts and fit in your externals when they are the most valuable. At the end of the day also keep in mind that there is a level of tank skill involved here and you can't always pull miracles.