Just clearing up some old achievements, and went into trine scythe on mythic and could not kill anything. Just to see I dropped down to heroic and barely was able to damage also. On normal it was fine.
I did de other side on mythic with no issue. Am I missing something about the rules on old dungeons?
Mists of Tirna Scithe is part of the current Mythic+ rotation - so the Mythic dungeon version is a max level dungeon until next season 
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Afaik there is no way to do the old Mythic scaled version until the season ends.
Ok hmm… and does that impact heroic? I’m just trying to do the dungeon hero achievement but heroic was pretty overtuned also
Not that I’m aware of.
Ok, thanks for the info. Ill prob give another try tomorrow
Don’t be afraid to ask your guild or someone if you need help. I imagine someone can muster the kindness on Christmas Eve 
The scaling also affects heroic. You can queue into the heroic dungeon under the specific dungeon section in LFD, though. Heroic and mythic 0 versions of old dungeons brought back into the current season rotation are scaled for max-level as of TWW. So, if you need to farm anything in Theater of Pain, Operation Mechagon: Workshop, or The MOTHERLODE!! make sure to do those before Season 2.
Editing to add: Necrotic Wake from Shadowlands is also scaled for the current season.
This here is why M+ should only use current dungeons.
Fortunately the heroics are in lfd so I was able to get it done