RESEARCH and DESTROY

The best place I found to do this was in the game's final level. You will have two opportunities when you are prompted to go through the arches where no enemies will spawn until you head through, allowing for instant turn skips.

And, as this is the final level, it's a great place to mop up the remaining turns before finishing the game.

There are a few levels where you are required to get every scientist into the ring of lights to continue the map. When this happens enemies will stop spawning until all 3 of your scientists have reached the designated location. You can then just leave your scientists out of the ring and continuously skip your turn over and over to grind out the turns required. I grinded this out in the Northern Frosts level at the point of the map where the goal is to "Run'n'Gun to the Jetties."