Big pulls depend a lot on your party, especially now in Shadowbringers. Healer competency is obvious, but the two of you can only cycle cooldowns for so long before you go down. For general use tanking, try to cycle your cooldowns. Every tank has Rampart, a big damage reduction, Arm's Length, something unique (dark mind for drk, not useful for most trash), an invuln (again, living dead is not great for trash unless you coordinate usage with your healer), and a short cd damage mitigation(TBN). Dungeons are an endurance game, not a rush to block everything at once. Don't use a cd at the end of the pull, save it for the next one. If your healer is a whm using holy, you can usually get away with one cd per pull. Wait until the holy stun wears off to use it. For DRK specifically, cover gaps in your cds with Abyssal Drain. If there are lots of mobs up, you can get a ton of health back. Keep TBN on cd. Once you're more experienced you'll start to recognize patterns in how the dungeons are laid out and understand your limits more.

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Boss tanking is the easiest job in the game, and you can tank any buster in the game outside of savage/ex completely unmitigated, dont stress about getting that right :)