It is possible to get behind the grills in the “Waste Disposal point” multiplayer level. There are two of them very conveniently placed next to the rocket launcher.
To get into the pipe (and behind the grill) you have to blow away the wall on top of the pipe with your rocket launcher or remote charges (You may have to destroy a bit of the wall next to the pipe so that you can jump up onto it.)
Follow the pipe while tunnelling further into the wall.
The pipe bends up after a little while and once you have destroyed the rock at the end of the pipe you should be able to drop down into it.
Once you are in it is very hard and frustrating trying to get out again. If you do have trouble getting out destroy some rock behind the pipe and it should be easier.
To get underneath the ramp in the “waste disposal point” multiplayer level (situated near the shotgun) you need to build a tunnel down from the left or from the back wall and take it out under the ramp.
You can then shoot anyone who goes past.
In “the Badlands” multiplayer level there is a secret “room” containing a fusion rocket launcher.
To find it grab yourself a rocket launcher and go to where you find the flamethrower.
If you’re looking up the hill there will be a little hole somewhere on the wall to your left. Enlarge the hole with your rockets and climb in.
There will be a fusion rocket launcher in the middle of the room and another hole on the other side.
In the “Warlords” multiplayer level you can build a huge trench across ground, which all the bots fall down.
This makes killing them easier. To do it get the rocket launcher and dig downward in the mountain that sticks out in the middle until your head is about at ground level.
Spin around until you are facing the middle of the level and destroy the land straight in front of you. Go all the way across the level until there is just a huge trench separating both towers.
The bots will just fall straight down ready for you to throw a incendiary bomb (from the flamethrower) or just jump down there and shoot their heads of with a shotgun!