there are several ways to do that. One is storing the grim reaper actor in a place where you can access for example your gamemode. then you can get your gamemode, cast it to your gamemode class and then get the variable with the monster reference:
other way can be adding a tag to your grim reaper actor and then cast it
other is getting all actors of reaper class and get the first found item (in the case is the only one in the scene)
anyway if you want to pick it when looking at I suggest you dig on trace functions
trace line for example will evaluate a line from a start point to an end point and if the line hits some actor it will give you a lot of information about that hit.
To make it working just be sure to turn ‘generate hit events’ ON and Block on collisions.
Good luck!
Dany