Here is a alternative to the drop key.
Place the following script into the Boss's Destroy Script Field
;Artificial Intelligence Script
;Header
desc = Leave corpse
;Triggers
;You can change settargetname=key to the name you gave the key. won't work with numbers
:state=0,plrdistwithin=40:settargetname=key,activatetarget=1,state=1
:state=1:state=2,suspend,coloff
;End of Script
And place this one in the key's main field
;Artificial Intelligence Script
;Header
desc = Pickup Key
;Triggers
:state=0:hudreset,hudx=50,hudy=90,hudimagefine=gamecore\text\pickedupakey.tga,hudname=keyprompt,hudhide=1,hudmake=display,state=10
:state=10,activated=1:state=1,playertake,coloff,plrsound=audiobank\misc\ping.wav,hudshow=keyprompt,hudfadeout=keyprompt
:state=1:rundecal=5
;End of Script
You won't see the key, but the Idea is to pick-up a key when the boss is dead and anywhere he is dead.