There are many ways to do this
I suggest creating a spawner object that you can move around the canvas however you wish ..either by using the
spawner -->set position to X{random(range)} Y {random(range)} expressions
or pathfinding
or bullet
or whatever you like
and either add a timer, a 'every X seconds' condition
or a system-->wait expression
to the spawn time ..
hope that helps somehow
When in doubt..or you are lost in a Zombie apocalypse or someone is giving you a hard time..Remember one thing..... Read the Construct 2 Manual..it will save your life.:)