I’m 7/7 (got the mount 2 weeks in MT). I also completed 8/12 classess in Legion.
I knew the mechanics from then, so it wasn’t really learning anything new for me. It was just playing flawlessly and not making mistakes, since it’s very tightly tuned. Attempting the challenges is roughly the same as attempting it with 895 - 900 ilvl gear back then.
Class I completed on & attempts:
Vengeance DH - 14 attempts (all in P2, died due to dps check)
Havoc DH - Wiped Xylem 15 times, Shadow 40 times (Only dps check)
Guardian druid - 30 attempts (P2 mainly due to not playing bear and not knowing the CDs really well)
Restoration druid - 15 attempts (mainly last phase, P1 is not so bad due to toolkit and arcane guys not one shotting)
Retribution paladin - 20 attempts (mainly due to not knowing the specc, mechanically really easiest)
Elemental shaman - 5 attempts (I was tired and I got killed by boulders, would 1 shot otherwise probably)
Restoration shaman - 40 attempts (Mainly in P1, since it’s really difficult for shaman. Last phase took maybe 5 attempts)
Fire mage - 2 attempts (would 1 shot, died because of boulders on attempt 1)
Frost mage - 15 attempts (The last 5-7 attempts just because I didn’t realize Raest enrages after Karam dies. This wasn’t the case in Legion, so I wasn’t really prepared for it).
WW Monk - 20 attempts (would do inside 5 if I knew monk)
Fury warrior - 16 attempts (15 with no consumables and nothing, then went to buy some old gear, unbridled fury, crusader, 1 shot it with that).
For me, shaman healing one, and Havoc DH were incredibly difficult and overtuned. For sure doable, but too difficult for my thinking.
Mad props for people completing holy priest, shadow priest, aff lock and boomkin ones. I wouldn’t attempt these no matter what 