First you'll face a doctor and a zombie human. These both have lots of health. The zombie's legs keep walking and then explode, so watch out for them. Take the platform to the very bottom. You can grab the 
over to the right if you want it. Head left, up the ladder, and over to the right. The blue worm curls up and explodes when you get near. Grab the minigun. Head down, left, then right, and you'll find an 
. Take the platform down to get the minigun ammo. If you ever need more just come back here. If it doesn't appear take the lift all the way up and then all the way down again. Anyway, take the first right then go up the ladder. Grab the 
and the 
red keycard, then drop down for some minigun ammo. Now head back the way you came. Stock up on some minigun ammo if you need to, then take the upper right exit. If you want health fight your way to the far right, otherwise take the ladder and head right to unlock the forcefield. Frag the zombie and grab the 
, then jump across for the three 
crystals. When you're ready drop down the pipe.
Head left across the gap and take the lift up for the 
, then go down and destroy the blue worm. Head down the ladder and kill the slime and the blue worm, then grab the minigun ammo and the 
yellow keycard. Take the lift down, being careful not to go to the very bottom as the blue worm there will hurt you. Stop about the same moment you grab the
small cross. Anyway, head left. Jump across to the upper platform for an X2, then take the other ladder up. Grab the 
and take the ladder down to the very bottom. Frag the zombie and open the teleporter cylinder.