There is a better way to blow the marked wall to enter the
  secret lab! First you must kill the first few guys at the
  first level and get a Dragon. Then go to the top floor to the
  wall with the "X" on it. Take out your Dragon and
  set it to the secondary mode. Now throw it at the wall and
  shoot it and it will blow up the wall! Now go in and finish
  the mission.  

  

  Easy Area 51

  Instead of lumbering the explosive box round with you at the
  start shoot it , it should say mission failed. Carry on as
  normal and when you get to the wall you need 2 blow up lay a
  DRAGON as a proximity Mine and shoot it the wall will blow up
  and it should say mission incomplete

  

  

When you start the game go into combat simulators. When you
  are in the combat simulator menu choose CHALLENGES if you open
  all of them you will have all the multiplayer levels

  

This works with players and sims. Go to  advanced setup.
  Before you play, make sure you have remote mines as a weapon.
  Strap a remote mine to a sim and have him attack a player or
  another sim. If he dies, blow up the remote and kill the other
  sim too. Or, you can blow it up when he gets close to its
  target. Its a very fun tactic when playing in a multiplayer
  game.

  

  Secret Ammo Crate

  In the first level of the game, make your way down the stairs. 
  Make sure you go to the bottom flight.  Then, crouch down
  and go underneith the last flight and you will see a very
  small ammo crate.
  

  

  

In any of the levels in the DataDyne Building go to the floors
  where you can look out and see cars fling.  Shoot the
  glass and gaze out over the night sky.  Shoot any of the
  cars with the CMP150 and they will blow up

  PS: Great way to kill people in the room next to you

  

  Carington Institute fun

  

  Go to the Carrington Institute and get the crate from the
  hanger and bring it to the shooting range.  Place the
  crate between the door opening to stop it from closing. 
  This way you can through knives at the two guys there. 
  Try out other weapons too

  

Ok, so i`m sure you all know about the floating block in the
  hanger right? Well get that and take it to the shooting
  gallery, put it in the doorway so that the door cannot shut
  properly. Go to the terminal and select the crossbow on any
  setting. (bronze gives you lots of time!) Go to the door and
  shoot your arrows at the man outside, he will not die but he
  will have loads of arrows sticking out of his body!!! This
  also works with combat knives, throwing them at him!!

  

This trick you need to do very fast because if you don't,
  you'll die from the gas.

  First start the level as always, but as soon as the level
  starts, press B to let go of Elvis. Then turn around and run
  up the slope, open the door to the scientists room where you
  last took Elvis out of the operating room. Walk aroun the
  scientists bodies and it will say" Picked up Falcon2",
  hit A to pull it out. Then run take the body out of the room
  and have a better advantage over the enemies.