I'm sorry, but where does it say that?
Gromok is never merciful. If an enemy target has less than 30% HP remaining he will deal massive damage to them, however if their HP is more than 30% he will deal regular damage to them. If an enemy is killed by Gromok's "Death Blow", (including summoned beings) then Gromok will be rewarded additional energy points.
As a result, he does not use it at all.