CIVILIAN MISSION 6 - THE CELENSOAN RITZ
The screenshot below shows one good setup to keep Venusian slugs away from the base in the Celensoan Ritz mission. Place one or two automatic lasers and a triple line of high-powered force field posts along each of the two canyons at the northeast corner of the map. That should fry the slugs as soon as they enter the map. Just make sure the force fields criss-cross the canyon, rather than forming a box. You want the slugs to pass through the fields.