Encountering Grover in long range open spaces - don't PEEK around the corner.
Good Grover players throw axes preemptively and you can take a lot of damage. Use short sprints between cover. This way it's harder to predict where to aim.
If the tip of his arrow shimmers, he is preparing an Impaler Arrow. Avoid being pinned to the wall at all costs; his Planted will do 2400 damage before you can even move again, and likely another 2400 to follow, killing anyone.