Easiest way to beat this fortification stage is stand by the ammo crate, throw one nade at the right window, throw one at the middle, throw one at the left, another at the middle, another at the left, ammo up, repeat.
A couple might get through near the end, so just let them lunge, adrenaline, and auto shotty to the head. Alternatively, you could whip out the RPG and alternate with nades. Haven't tried this method personally, and don't like the weapon taking up half my right side of the screen, but you could use the RPG reload/grenade throw trick to push out extra asplosions. Just make sure to swap out to a shotty or rifle if they get through.