I defeated the second crab-like boss a number of times but am now totally stuck as to what to do. I know I have to use the missiles, but on what? I've spent a very long time trying to figure this out to no avail. Any hints? Thanks.

I bet you can use the rockets on a blocked passage you came across earlier in the game. You will find it, just be patient.

Just go back. Double-thick rocks you may find.

Are you registered? If so, try going to the factory. Also there's a hint button above the map when you press escape.

Okaay... I'm looking for these frikkin' rocks but not finding them... need more help...

Wherever there is a blank spot on your map, check to see if it is a path that you now can blast open. There may be considerable backtracking involved.