Global Variable Timer number Default value 0
Global Variable SpeedInc number Default value 1
System > Every 1 Sec
.. Create object > enemy1
.. Set bullet speed to SpeedInc * 100
System > Timer = 20
.. Add 1 to SpeedInc
.. Set enemy1.bulletspeed to SpeedInc * 100
System > Timer = 30
.. Add 2 to SpeedInc
.. Set enemy1.bulletspeed to SpeedInc * 100
For the Timer, you can also use the Timer behavior (set on Regular type) that could add SpeedInc value to itself every 10 secs
On timer "SpeedInc"
.. Add SpeedInc to SpeedInc
.. Set enemy1.bulletspeed to SpeedInc * 100
As long as you use the same action when you spawn a new instance and when you raise the speed, all instances will have the speed applied to it, and so no need again for the instance variable, since all instances must end with the same speed.