All the challenge goes away after the 30th floor. Money no longer is a problem, and gear gets immensely overpowered for there to be any luck or planning involved.
I found the most effective way is to be a rouge and try to get the revolver as early as you can. Then upgrade it's fire rate by 2 and then reload rate. Take care of ground enemies, revolver even works good on armored, while your partners watch both air and foreground.
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