You need to find the angle between the monster, and the player, and then just switch the images by texturing a plain with a different angle. I don't know which of these 3 functions you would use, but you can try them out, you only need one of them.
function xang_between_objects(o1,o2)
y1#=object position y(o1)
z1#=object position z(o1)
y2#=object position y(o2)
z2#=object position z(o2)
ret#=wrapvalue(atanfull(y1#-y2,z1#-z2#))
endfunction ret#
function yang_between_objects(o1,o2)
x1#=object position y(o1)
z1#=object position z(o1)
x2#=object position y(o2)
z2#=object position z(o2)
ret#=wrapvalue(atanfull(x1#-x2,z1#-z2#))
endfunction ret#
function zang_between_objects(o1,o2)
x1#=object position y(o1)
y1#=object position z(o1)
x2#=object position y(o2)
y2#=object position z(o2)
ret#=wrapvalue(atanfull(x1#-x2,y1#-y2#))
endfunction ret#
