I did this recently with an special enemy that is spawned in an specific interval of time (3~8 seconds).
Set Enemy to floor(random(3,8))
"Enemy" is a global variable that is used in a later event that interrupt the commom enemies spawn.
System > Set group "commom enemies" deactivated
Generator > Spawn Special Enemy on layer 0
System > Wait 1 second
System > Set group "commom enemies" activated