Definition of a Boss Mob is surely one that is Harder (much) then the normal Mobs around it. The Stats I have for the last two are means its Def a Boss, last Spikey dude, only One spawns - is that not the sign of a Boss Mob?
Here is Any Idea - why dont you Stick EvilMir from Mir2 in that "Hole" if thats what your after...you will have to sort the graphics out on the mob a Bit but just make it so it never moves...