There's a way to create a room which you can only stay in for a certain time period. After this, next stage comes - regardless if the character passes the "bound"-border or not. I use this to make an armoury.

First you create a normal phase with a "bound" of 790. If this value is bigger, it'll create errors, if it is smaller it doesn't look realistic. In this phase, you should note everything that should appear in the armoury. In the second phase, make the "bound" value -1000.
Now, the stage will continue directly to the next stage without giving the player a chance to act. Because of this, you need a "Timer", which also should be included in the first phase. The easiest way to do this is to let a bunch of enemies appear one after another without any lifepoints at an unreachable place. They die immediately - but they need time to fall to the ground. The timer in the following example lasts for 25 seconds.

<stage> id: 0  #stage 1-1
        <phase> bound: 790
                id: 101 x: 400 ratio: 3.0 #hoe
                id: 120 x: 400 ratio: 3.0 #knife
                id: 121 x: 400 ratio: 3.0 #baseball
                id: 122 x: 200 ratio: 3.0 #milk
                id: 123 x: 200 ratio: 3.0 #beer
                id: 124 x: 400 ratio: 3.0 #>
                id: 150 x: 600 ratio: 3.0 #stone
                id: 151 x: 600 ratio: 3.0 #wooden_box
                id: 300 act: 60 hp: 250 x: 150 #Jan
                id: 30 hp: 0 x: 2000 times: 39 #"Time"
        <phase_end>
        <phase> bound: -1000
        <phase_end>
<stage_end>

To see this in a stage I created this study example:

Armoury Download now!

Inspired by MH-Razen