Infernape's better defenses don't really matter, it's still going to be OHKO'd by anything that has a strong STAB attack. But then again, so is Zangoose - and Infernape is quite a bit faster and has a better movepool IIRC, so it has a better chance to sweep the opposition before it takes that one hit.
As for your particular Zangoose: As people have already said, ANY steel type will completely shut you down, as will Shedninja (even though you're unlikely to meet one). Any sweeper/tank Pokemon that can take a hit will also murder you, since your Zangoose most likely can't.
Try something like this:
Switch out Crush Claw for Return - it's a much better move, and you'll never benefit from the potential defense drop anyway.
Also try to get Brick Break or Close Combat in there - you'll need something to take out those Fighting and Steel types. Fire Punch is also something to consider, if you run into a pesky Grass type.
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