Anybody know the best weapons to beat the harder levels?
I don't think there is a "best"
I like fully upgraded minion and medic minion with a troll in front to stop incoming enemies. Harpoons in the area of this little roadblock are good.
As are spike tiles in front of the troll. Acid drippers over ladders or gaps coupled with a/c units are a good health drainer.
Anything that slows down or stops the incoming enemies and a good damage drain will always work. Treadmills, a/c, acid drip all along a floor will take out most enemies even.