If a Grover is by himself, this can give any champion with decent mobility a chance to close the distance between them and Grover. The closer you get, the more damage you do and the less damage Grover does. Don’t forget about the vine though!
Nullify makes BK completely useless. For talents if you know the how good the enemy BK is (from previous matches), BK is the only dmg or your other talents would be useless in the match pick the Direct Current talent to make it harder for BK.