One brute is easy to take down but trouble starts when there are 3-4 ganging up on one person (particularly during ship boarding) is there a way I can defeat then easily at this time
Use Smoke Bombs, which will make them no longer be on combat state, then assassinate them (they can't defend against assassinations).
You can also shoot them twice (kill) or ,to conserve ammo, shoot once (stun) and then attack.
Regarding Break Defense:
It's also a good method, as Nzall has mentioned. However against groups of Brutes it can prove to be dangerous/impratical as while you're finishing one off after breaking his defense, you'll have another hitting you, thus saving his mate