You need a condition that won't be true after the player passes the level.
I can't say what the easiest way for you will be, but I think I'd do something like this with the level conditions:
| Trigger |
| Conditions: |
| ¤ Current player custom score is exactly 1. |
| Actions: |
| ¤ Do level #1 stuff here. |
| Trigger |
| Conditions: |
| ¤ Current player custom score is exactly 2. |
| Actions: |
| ¤ Do level #2 stuff here. |
| Trigger |
| Conditions: |
| ¤ Current player custom score is exactly 3. |
| Actions: |
| ¤ Do level #3 stuff here. |
Etc. (One for each level)
Then, to switch from one level to the next, I would only need to detect when the player beats the level (Probably bring a zergling to X location) and add 1 to the custom score.