Add trigger, put your bolder in a blueprint, get ref to trigger in level bp, get on overlap begin event(overlap has to be activated on both actors), spawn your “bp_boulder” from class.
For the location you have many different ways. One is to place any object in level, where you want the boulder spawned.
Remember the xyz values and enter them in actor spawn loc. Use get actor loc. Enable physics in bp_boulder for static mesh.
For the “Hazard” part you need some health before you can subtract anything… https://www.youtube/results?search_query=ue4+health+tutorial
But i would put the bp_boulder at the cliffs and activate physics, when triggered.