hired gun

Short definition from Wiktionary

Noun

1. 1
   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 their employer.
2. 2
   A person who is employed to advance the interests of their employer, especially in a vigorous manner using such methods as political lobbying, legal advocacy, or persuasion.