When entering any area that has several monsters of any kind look
for green barrels containing toxic waste. Take out any gun and
shoot the barrels to wipe out enemies quickly. Note: Standing too
close to a barrel of green waste and shooting it can injure or
kill you as well. Try to stand back when firing at those barrels.
Also, it usually takes more than one shot to blow up a barrel from a distance.
You can find this area in the Toxic Refinery (Episode 1, Level 3). Complete
the first half of the level (before you enter the blue door), then go back
to the starting point. Look inside the pool of radioactive waste and you
should see a platform (if you flipped all the switches in the first half).
Walk across the platform towards the wall it leads to. The wall will open
and you will have to defeat a series of enemies. Explore the secret area
more and you should find an exit door. Enter it and flip the switch (the
stats will not appear). You will now be an area called the Military Base.
Note: This area can be very difficult.