Players:
(Player)
Conditions:
Switch #(any switch) is set
Player commands 0 men(or specific unit)
Actions:
Wait 5000 milliseconds (<-- if you dont want them to appear as soon as they die)
Create 1 (unit) at (location)
Preserve trigger
Then when they get 25-30 kills..
Players:
(Player)
Conditions:
Switch # is set
Player has # kills
Actions:
Clear Switch #
Then he wont appear anymore unless you set the same switch again