If you want to create invisible walls, the player can't pass you have to use "bounds" smaller 794. But I wouldn't use this effect in the first stages - cause a blinking character can pass the wall and will be thrown back when he comes into normal mode.
It's only effective if you use "bounds" smaller 250, so the bigger part of the screen is impassable. To let the enemies appear out of the screen, you have to take a higher x-value.

<stage> id: 1  #stage 1-2
        <phase> bound: 150
                id: 30  hp: 50  x: 800
        <phase_end>
        <phase> bound: 900
                id: 31  hp: 50
        <phase_end>  
<stage_end>

A good possibility is to create an armoury first, then use the wall effect in the first phase of the next stage and let appear a bunch of attacks. After this the enemies come.

To see this in stage, you can download this study example:

Wall Download now!

Inspired by Kate