Go all the way to the armory in co-op mode. DO NOT I
REPEAT NOT GO INTO THE SHIPS FUSION REACTOR. Once your
inside the armory have 1 person go into the fusion core.
and what's this? the only enemies that appear are those
annoying little infection forms, no other flood, or
sentinels. just you and the monitor. once you do all the
reactors have your friend come in and the enemies will
appear. cool huh?