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: 
***Unconscionable*** is a legal term to describe just such a contract or deal.

> 
> 
> > 
> > In contract law an unconscionable contract is one that is unjust or extremely one-sided in favor of the person who has the superior
> >  bargaining power. An unconscionable contract is one that no person who
> >  is mentally competent would enter into and that no fair and honest
> >  person would accept. Courts find that unconscionable contracts usually
> >  result from the exploitation of consumers who are often poorly
> >  educated, impoverished, and unable to find the best price available in
> >  the competitive marketplace.
> > 
> > 
> > 
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> Contractual provisions that indicate gross one-sidedness in favor of
>  the seller include provisions that limit damages against the seller,
>  limit the rights of the purchaser to seek court relief against the
>  seller, or disclaim a Warranty. State and federal Consumer Protection
>  and Consumer Credit laws were enacted to prevent many of these
>  unconscionable contract provisions from being included in sales
>  contracts.
> 
> 
> 

([Legal Dictionary - The Free Dictionary](http://legal-dictionary.thefreedictionary.com/unconscionable))
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.