Mission 45

First you need to kill all the spawners at the top of the level to lower the Gate's personal shield. After that you should go down the leftmost pit, avoiding the trap, and destroying the shield generator. From there you can easily destroy the Gate. If you setup your Graviton Turret above the middle pit before you destroy the shield generator, then you can activate it to start immediately destroying it.

Tip:

It is easy to kill the Overlord on this level by using the pink traps properly.