https://gyazo.com/cca6bb50db22f08e21e07604b469b947
These are the ones I've completed so far, with a "guiding" difficulty - Some of them are extremely based on my gear and my lack of experience when I did them.
I 1shotted the - Balance Druid / Unholy Death Knight / Fire Mage / Elemental Shaman, all of them I had done the challenge on other specs though, so I knew mechanics and whatnot.
Some of the "harder" ones for me was due to lack of gear. I don't have Thrash bracers for Feral / Corruption Ring for Affliction / Whirlwind belt for Fury Warrior, those items can actually drop the difficulty from upwards of 9 and all the way down to a 3/4 imo.
Generally the healer ones are the hardest ones (I've heard. Just recently started trying them out and apparently they scale with gear unlike all the others?) - Tank ones are also generally very difficult (especially the specs that have a hard time with self healing, Guardian Druid / Brewmaster Monk / Protection Warrior).
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Now that I've talked way to much about that which has nothing to do with the topic, these are the skins I felt like I just had to have (not in order) :
Unholy Death Knight
Guardian Druid
Feral Druid
Affliction Warlock
Havoc Demon Hunter
Windwalker Monk
Protection Paladin
Retribution Paladin (Only cuz it's my main, don't really care for the skin)
Outlaw Rogue
Demonology Warlock
Protection Warrior (Soon my friend, soon)
Yes it's a big list, but I actually think that most of the skins are at the very least decent, with many of them being straight up top tier.
(FeelsBadMan hunters)