hired gun

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Slang One, especially a professional killer, who is hired to kill another person.
2. n. Slang One hired to fight for or protect another.
3. n. Slang One who is proficient at obtaining power for others.
4. n. Slang One with special knowledge or expertise, as in business, law, or government, who is hired to resolve particularly difficult or complex problems.

Wiktionary

1. n. A person who is employed as an armed guard, enforcer, or mercenary and who is prepared to use gunplay or similar violent methods in order to assert the interests of his or her employer.
2. n. idiomatic, by extension A person who is employed to advance the interests of his or her employer, especially in a vigorous manner using such methods as political lobbying, legal advocacy, or persuasion.

WordNet 3.0

1. n. a professional killer who uses a gun