Just throw.
Or, alternatively, if there has to be some kind of aim assist... Slightly highlight a spot on the ground (RMB), click, and V then tries to hit the spot based on REF/skill (there could be a difficulty indicator on the highlight; a number or a colour code). The grenade is then thrown at the general direction that’s being aimed and hits or misses (in a random direction) the spot based on a check, and blows up where it lands.