I think you just need to finish a bounty of winterspring to get that task done
had the same thing with rokara, and after getting a winterspring bounty with her, the task was done
I would advise you to have at least 4 level 30 mercs if you want to compete in winterspring I found it very easy to do with rokara, samuro, grommash and xyrella
It was a bug that got fixed yesterday that was preventing completion.
Unfortunately, you'll get blocked in winterspring pretty hard if you didn't get lucky opening any mercs with decent fire abilities. Managed avalanchion (barely), but Ahune is very rough. Doubt training up abysmal hunters will be of any use whatsoever (explosive trap against mobs that rarely attack!), so I'm grinding for coins to craft any fires, then going to have to grind one of those to 30 and get enough coins for upping at least one ability.
Other than the typing issues, L3 abilities and L30 should see you through winterspring, I'm finding, other than those cases where 3-4 enemy minions all lob spells/taunt ignoring abilities at one of yours for two turns.