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Considering Disco ran with Mage or Rogue back then that should not be an issue.
Also you're not casting Mana Burn when the enemy DPS is on you, you're casting it while your own DPS puts pressure. It's no different from Disco spamming smite in current, it was a spell you used when you had the upper hand. And with a Frostmage/Rogue's control that wasn't much of an issue in 3.3.5.
The deal was that every healer hit like a wet noodle in 3.3.5, so having mana burn was huge because it allowed you to play offensively, something no other healer could really do.
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Redactedmane's top 100 is full of Priests so I'm gonna press X for doubt.
Considering how bonkers Frost Mage was in 3.3.5 you definitely can pull it off.
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At high level, with 1400 resil? Fat chance.
Dude, literally please stop talking and posting everywhere. DHs ARE OVERPOWERED right now and its a fact. Just stop.
Don't tell me what to do.