I think the game should have a tutorial mission for training and getting used with the game, regular weapons (including the grenade launcher), and reaction fire. Instead of your squad it could feature other soldiers, like sniper, explosive expert, close combat master, etc.. and several aliens, or you could use your squad but the soldiers can't be killed for real (get revived after the training mission).
Other suggestion is to build a base facility like a simulation room, maybe with a "neurolink" or something like that, where soldiers experience custom battle scenarios from a chair (like in the Matrix). The simulated battles should give some experience, but not as much like a real mission. Or there could be other kind of limitation, like stress from the neurolink, so you can use the simulator only once per week. The player could choose the battle condition, how many aliens, weapons, civilians, landscape, etc.