bear traps, caltrops, mines
throw toxic gas when you enter during an invasion, before the fight starts (or wait until it does, since you used all those bear traps)
I haven't tried it yet, but I had an idea to just surround the landing zones with rings of bear traps, that way you can get close to throw grenades or whatever you do. My method of putting down all those bear traps and caltrops over the beach kinda backfired when I couldn't really get to the crossbowmen who were sniping the troopers. Also, throw flares so your dumb trooper buddies can actually hit something.