I just did this with a buddy, we failed the first try and got it at 84% on the second try. I did machine guns with sniper turrets and dampers. I also built a repair station by the base. He had machine guns and snipers and just concentrated on putting flak placements anywhere he could; no upgrading, it seems more bullets is king here. I had dampers lining each path at the end and we literally had no place to drop any turrets at the end.