ok, i've been stuck in the last checkpoint (mortars) for 2 days.
sometimes i'd run out of ammo, or get a mortar into my skull.
i ALMOST threw my controller into a wall (god bless wireless! does this work as a boomerang?)
anyway, i found a MUCH easier way to kill all those mortar guys: Use your machinegun. just tried it now, and worked right on the first time. i used the MP44 ant i could kill them easily aiming with LT, and saved the sniper bullets for those who were real far away. Easy.
hope this tip helps some of you guys.