In certain levels as soon as you walk into a room about 10 guys
see you and run in different directions.  An easy way to kill them
all is before going into the room hide behind a wall and toss in a
regular grenade.  When the grenade explodes it should kill them
all because they might not notice it.  If any are left just shoot
em of you could through in a flash bang grenade and run in shoot
them (only do the flash bang one if you have low health).  If
multiple enemies are behind an object through a grenade at em but
make sure it goes BEHIND the thing they're hiding behind.  If the
grenade explodes in front of the thing no damage will be done to
your enemy.