When you play on your level (meaning trying to time it, not 2 chest it), packs take a bit more time and you run out of stuff since they anima slash more often.
On a 12 you are dead as bear without using a defensive since there are 2 of those. And the cast time is 0.5 seconds. You can not react. (not an exageration. it is 0.5 seconds Soul Split - Spell - World of Warcraft)
I know when it happens but it happens so often that you need multiple externals. I got 1 shot myself on a +6 by it already. I know i am not 630 ilvl, but still.
Tankbusters are good but the frequency, the amount and the dmg type arenāt, if you put those 3 categories together. There are too many magic dmg tankbusters. 3 out of 6 tanks just die to it.
Plus thereās even a few 3 caster mini packs, in e.g. DB, GB. That said, pulling 1 pack at a time, will get you through lower keys maybe, but higher I doubt it. I just started tanking again, doing some chain pulls, itās not easy but needed. But you also need to rely on a team that interrupts etc, I usually /focus an important one, but I can interrupt them all.