In this mission you must defend three artillery pieces in a wide open arena.
You will face many waves of Volt Droppers, explosive Knobs and Cathodes, Artys and Big Willies as well as waves of regular Tommys, Resistors and aerials. Basically the enemy throws everything they've got at you and you will struggle to spread your firepower between the three locations in order to keep them from being destroyed.
For a choice of Trench, it is a trade-off here between being able to quickly move around the massive area with a lighter Trench, or having a heavy loadout of long-range weapons which you can use to reach out to massed enemies from afar. If you choose a heavier Trench, ensure you select a Sniper-type weapon for yourself that you can use to kill the Volt Dropper.
For emplacements, a good choice for this mission is Sniper Turrets which are the best way of quickly destroying the waves of Volt Droppers, who can kill your structures fast. You should also accompany each Sniper Turret with a couple of Machine Gun Turrets that can defend it from the waves of knob/cathode suicide explosives.
Overall this is a challenging and varied mission, while trying to beat it, many players will play it over and over trying different strategies and mixtures of Trench, weapons and emplacements. It is especially difficult in the beginning when you do not have a lot of scrap to make defences. A good strategy is to strongly fortify one structure on the outer edge with a big set of emplacements, allowing you to spend most of your time between the other two structures, actively defending until you get more scrap to build out defences at each structure location. This should limit the amount of running you have to do from one side of the map to the other (which you can do a lot of in this level).
It is difficult to make it through this mission even at bronze level, let alone to achieve a gold medal rating (80% building hit-points remaining). This mission is also one of the most difficult Survival Mode achievements in the game, where you must reach the epic level of 30 waves.