Re: Smash in head/smash head
As has already been pointed out, context is important. There are several variations I can think of but they really would depend on what was involved! As you will see, different prepositions are involved each time.
A tree was struck by lightning and crashed down on the man, smashing his head. (The tree crashed, but the effect on the man was that it severely damaged his head.)
There was a huge fight outside the pub between a 22-year-old and a 45-year-old man. The older man is now in hospital after the 22-year-old smashed his head in. (Note "to smash someone's head in" is definitely an informal phrase, probably only used in BrE.)
The burglar used a baseball bat to smash the homeowner around the head and face.
The tree fell on his head.
The tree crashed (down) on his head.
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.