Phrases:
* Do someone's dirty work
* Be someone's fall guy
* Be a pawn
* Be a tool
* Be a front
* Be a scapegoat
* Be used
Words:
* Proxy
* Stooge
* Shill
* Go-between
* Agent
* Runner
The best choice depends on the specific context and the nuance you want to convey. For example, "pawn" suggests someone being manipulated, while "scapegoat" implies taking the blame for someone else's actions.