Mission 21

The bluetonium should be in the 3rd spawner down from the top. If you don't have the Portal Generator then you need to destroy the shield generator on the bottom right of the tower (you can do this by blind shooting from the top of the tower.

From there, the pattern is to go to the left side of the tower, destroy the spawner and shield generator, then go to the right side of the tower and do the same. The shield generators on one side open up a section on the opposite side of the tower.

Riding the Graviton Turret up the down can be done multiple times and is faster than climbing. Just be careful not to get knocked off by enemies.