Actually it only bugs if the worm is underground.
It bugged for me a few times and all of them matched the condition.
On topic:
You can see my progress on my Signature and if anyone reading needs help you can ask.
The mage tower is a wonderfull thing, i would've never played my alts like i've been doing for the last 3-4 months and i don't even touch Mythic+ or Normal Antorus(personal traumas) and many of them i didn't even had the correct legendaries for some of them.
The most important part is learning the fight, if you do, the gear you get over time will only make it easier and i wish more would try the challenges.
Also to the OP:I did one shot the Enha Shaman challenge, as a 929 shammy, after doing every other Sigryd challenge.Does this count?