Witherbeasts are enemies with a blue body and purple crystals on top.
They heal other enemies when they burrow, emitting crosses that allow you to know they’re healing. They typically will not burrow until they reach the objective the players are defending, i.e. a crystal cart.
I have found that playing the Harbinger on Normal is the best place to farm these kills. To set up, switch to the Monk and set down fire traps in all three lanes, as many as you can fit. Then, beat the first stance of the Harbringer. In his second stance, the enemies that attack will come in waves every 30-60 seconds or so, and every wave will come with a Witherbeast that can easily be killed. Continue to farm the kills without damaging the Harbringer to have this loop over and over. Using this method, all 50 kills can be done in under an hour.