Give bounty/bosskiller troops four traits.
Then a throwaway troop would be useful, and people would spend the gems to mythic them 
It was a mistake to take something away to add the something special to a troop just to accommodate a new play mode. Balance a troops three traits and spell first, then add the bounty/bosskiller ability as a bonus.
1 more spending of gems on event troop
2 less complaints about throwaway troops
3 underwear?
4 profit!