Marines really could use something else than a large orbital bombardment, but something similar. the firing would whistle, warning aliens.
A mortar-squad would take orders from their SL and he would designate targets to the squad. The mortar would come pre-equipped with somekind of a targeting map of local area, preferrably the 40 tiles to given direction and a simple click there would fire a round of mortar shelling. The firing would give away a whistle before it hits the ground.
The mortar would preferrably need two marines, but one will work well. Other one would load and the other would be the user of the mortar. Once they have deployed the mortar, the operator of the mortar looks throught a special zooming goggle.
