Check my tower defense demo for an idea how to do waves: viewtopic.php?f=4&t=2807
Basically, you need to do the following:
- define a condition for next wave to spawn (all enemies defeated - no enemy objects on screen, time expired etc.)
- condition that checks whether the spawning is still active (for multiple enemies) - if the spawn is not instantaneous, that is
- create your wave as you want it (in your case, you could use X offset from the midpoint to spawn them along the edge)
- finalize the spawn process (reset the conditions, add to the number 'Wave' or whatever)
In my case, I decided each 10th wave to be the boss wave, all others are spawned in increments, one after other, from the same spot.
Hope this helps you.