So, this is another cry story, but I truly want to learn from it so please enlighten me 
Was making my way through westfall as a level 13 gnome mage when I with full HP and mana engaged a level 15 Goretusk.
The pig resisted some of my frostbolts and therefore I kited it a bit, while having a little less than half of my HP I was a bit greedy and thought just another frostbolt would end it, but not just the amount of damage.
However, at the same moment as I was prepared to just take a hit and run I got hit for 106 with a crushing blow.
As I was so surprised of the damage I had to google when crushing blows occur and everywhere I read it says 3 level above.
So, how could I get a crushing blow?
Screenshot: https://imgur.com/PVjBqgQ
The piggy: https://www.wowhead.com/classic/npc=157/goretusk
https://www.reddit.com/r/classicwow/comments/1641f1k/beware_of_the_goretusks/
Goretusks charge, after that deals 2-3x damage with Crushing Blow, on a clothie with low defense skill and potential crit = death.
Alright, thanks a lot.
I will definitely avoid all boars as a clothie if I go agane… 
Your defense skills also comes in play, if you would only have 1 defense skill and you are lvl 13, most hits would be a crushing blow, now you dont usually have only 1 defense skill at lvl 13 but due to the mob beeing 2 lvls higher and you are prob not having full defense skill for your lvl its easier for the mob to crushing blow you.
I normally end up on an hunter to go and train defense skill at lvl 30-40 becuase I normally dont get hit much and that result in me having a low defense skill so the mobs was always crushing blowing me and if I wanna tame a pet, non elite or not (Yeah freeze trap EZ MODE) they will hurt.