Mission 29

This is much easier with the Portal Generator. If you don't have that, then you need to ride the platform to the right part of the level. You must then ride the platform there to the top of the level - the green portals will take you to the bottom of the level (likely killing you) and so they must be avoided.

In the middle part of the top area is a portal. You can jump into this and destroy the shield generator in the area it brings you to, but before you do that, go to the far left and make the platform there go to the bottom of the level.

After you've destroyed the shield generator and spawner, take the platform back up. The room at the top left has opened up and you can destroy another shield generator and spawner.

This drops the shield at the top of the level and lets you drop into the room with the mainframe.

From there it should be fairly straightforward.

Tip:

Send the platforms to the bottom so you can make your way back up if you hit a green portal.