Yeah I did plenty of that, needless to say. But you can arrange to have a checkpoint just before you turn the corner and see the Grunts, so that helps.

: I played around with this battle on the PC version with infinite invisibility
: several times on legendary difficulty . I noticed that the Grunts have a
: higher degree of success winning this battle if the marine combat form
: with the shotgun doesn't show up.

Doesn't show up? On my Xbox he always shows up. You get the shotty guy, the plasma rifle guy, the carrier, and a few loose spores. Is PC genuinely different, or did you perhaps merely overlook the shotty guy on some occasions? Often the combat Flood are initially hidden around the far corner, near the exit door.