Well, you could use this destroy script:
;CoffeeGrunt Scripts
;Header
desc = Fade Out Disappear + Inc Var
;Triggers
:state=0:state=1
:state=1,alphafadeequal=100:decalphafade=0
:state=1,alphafadeequal=0:state=2,globalvar=1,incvar=1,destroy
;End Script
..and edit the value in this script, so that when that number of enemies has been killed, you can finish the level, make it invincible to damage and assign it as objective 1, (make sure that you have no other objective 1s in your level):
;CoffeeGrunt Scripts
;Header
desc = Fade Out Disappear
;Triggers
:state=0:globalvar=99,state=1
:state=1,varequal=XXXX:destroy
;End Script
Change the XXXs into the appropriate number.....