You could:
Create a sprite, call it timersprite, set it invisible and give it the timer behaviour.
system on start of layout (or after start button pressed or whatever event you want) - timersprite : start timer duration: 30 tag: "enemyspawn"
add another event:
timersprite : on timer "enemyspawn - create object "robot" on layer 2 (x:12751,y:430)
- timersprite: start timer duration: 30 tag: "enemyspawn"