Destroy Big Bessa without anyone being hit by its ram or cannon at level 90.
Big Bessa will fixate on a random person and will start chasing this person for a few seconds. While it's chasing this person it'll hit with its melee attack (if in melee range of course) and an easily avoidable cannon (a red circle will appear where the person that is being chased is standing). You have 3-4 seconds to move away from it.
To get the achievement you must destroy Big Bessa without anyone getting hit by it.