First, throw a frag grenade into the
fray, and make sure that it doesn't bounce off one of the charging ones, and
back to you. When it detonates, that should take out a good three to five of
them.
Then, use the shotgun on the rest. If you run out of shotgun ammo, and have to
reload it, then switch to AR, and pump 'em full of lead. Burst shots are better,
if you have the luxury. Luxury = Range/Distance from target.
You don't want to get surrounded, so keep your distance when tackling large
groups, but not so far that you have to use a sniper rifle, which goes through
flood with no effect. If the group gets bigger, then throw another frag.
But DON'T use plasma grenades. I learned my lesson a million times over when I
stuck a plasma to an oncoming flood. BOOOOM!!! 1 Mc dead, 7 flood dead.