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.
I believe the word you are looking for is 'zero-sum'. The notion of zero-sum [has its roots in game theory](https://en.wikipedia.org/wiki/Zero-sum_game), but the concept of a "winner-take-all" outcome to an interaction between entities can be found all over nature. For example, when an ecosystem has reached its carrying capacity, the survival of a group/individual comes at the perdition of another group/individual.

If you wish to explore more abstract, universal manifestations of zero-sum circumstance, see also [Conservation Law](https://en.wikipedia.org/wiki/Conservation_law).