Use Team Awareness To Get Through The Saboteur
It's all about having those eagle eyes. Keep one person planted around the defense point at basically all times, with everyone else looking out specifically for enemies spawning in around the edges. The person in the middle should be capable of killing anything that gets close very quickly, so a melee character is pretty good there. However, the person in the middle can't just stay in the middle the whole time and not contribute, so it requires a lot of mini-map reading and a lot of awareness. It's tough, but if you bring strong characters and just look everywhere for stuff, it's not so bad.