I've gotten to the level where I can start going through some basic Japanese material and largely understand all of it, but there are some times where I'm confused by particle usage, and I'm not sure if it's due to colloquial contractions and omissions.
In this case, I've started to go through the game Mother 2, and although I can understand what the following sentence means overall, I'm not quite sure how the particles are used.
For context, the Onett city police chief is talking to the main character, who just beat some policemen.
The entire sentence is [オネット警察のつわものを相手に、これほどの力を見せるとは…]
Specifically, what is happening with the を相手に part?
If I remove を相手, making [オネット警察のつわものに、これほどの力を見せるとは…], the meaning of the different particles is easily distinguishable, basically translating to [To think that you could show our strong policemen such power..]
What does the を相手 do? As far as I can tell, the sentence is making つわもの a direct object, but 相手 is a noun. Is して being omitted or something?