I just did this on 10 man heroic today. Soloed on prot pally just stood on stairs and waited for necrotic plague to jump from me to mobs and back around until it reached 30+. killed Arthas got achieve. Everything worked as intended. Did not move at all did not kill any mobs. Didnt get knocked back because i was on stairs. So just stand and wait.
Would I get credit for the 10-player version if I do the 25-player? It sounds like I’d need to do this twice.
Seeing all of these high level characters in the 90s and 100s makes me sad. 