Time to use some big explosives.  Grab some of the C4 off the table and walk over to the artillery a bit further down the path.  To use the C4 just select it from your inventory like any other item, then use the left and right mouse button to set how long until it explodes.  Press Enter to set the C4 on the ground then wait for it to explode.  Once the artillery is toast continue up the path to the next Training Area sign.
