Question: I'm trying to come up with a way to express the opposite of "**mutually beneficial**" in a way that is just as efficient. The idea I'm trying to concisely express is that a deal, agreement, contract etc. benefits only one party instead of both.

Answer: 
One might say the deal was ***asymmetric*** or ***one-sided***.

*Asymmetrical* — [M-W](http://www.merriam-webster.com/dictionary/asymmetrical)

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> having two sides or halves that are not the same
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In either case it seems clear that one party is getting the better end of things.
A one-sided arrangement can create a relationship that is [commensal](https://en.wikipedia.org/wiki/Commensalism) if it does not harm or benefit the other party, or [parasitic](https://en.wikipedia.org/wiki/Parasitism) ,if it does. "Commensal" would be unlikely to be understood outside of specific academia.