Can't complete Backwash because of monster not targetable

It seems like the monster respawned half-underground and couldn't be damaged (I tried with different skills), but I can still take damage.

By the way, I can't use the stash properly in the Trial of Chaos and Trial of the Sekhemas (with my controller). When I press "Y" (Xbox console), it drops items rather than moving them from pack to stash (or vice versa).

I cannot complete the map either on PC, 2 enemies left, no sign of rare monster.