Good to hear. That trough is definitely the place to go if you like things cataclysmic.

: Did you try standing back behind the marines and watch them fight without
: your assistance? Sometimes the marines can be pretty smart and actually
: win against the enemies. It may help to take out a few grunts to keep them
: from tossing plasma grenades at the marines.

No I didn't try it, but I can tell you now that they wouldn't stand a chance unassisted. Grunts would soon decimate the blockade (that's assuming the Marines don't do it themselves by fumbling frags), and although they can fight on for quite a while, they'd be unlikely to prevail (on Heroic at least). And even if they did, it would be mostly due to the chain-gunner.

However, I've just tried a quick experiment. If I stand back and restrict myself to sniping Grunts, there's a small chance of the Marines prevailing. But it's hard to snipe quickly enough to prevent a throw (especially in view of the framerate slowdown, which is quite strong to begin with).