Does any one know how to fix a bathtub faucet?

My tub faucet keeps popping off the pipe it is inserted to and hitting my legs every time I take a shower.

This is will be the third time I asked the landlord to fix it. His solution is caulk. Which maybe holds for one day if I don’t use very much water pressure, otherwise it pops off and shoots me in the back of the legs.

There is a pipe that is longer then the faucet attachment, it comes out of the wall at an angle and there is no ring or anything to screw it into. There is a textured shower gaurd and I have no idea what is behind that.

What do I need to do to have a shower that is usable?