Easiest way to get the achievement, once you unlocked Hazard 5 on Azure Weald, is to do a weapon mastery on Hazard 5. Weapons like the LOK-1 Smart Rifle will put you on Azure Weald - you can check the right info window on which level you will play.
During a weapon mastery level you only have 3 stages to complete and this way you can completely overpower one weapon and easily kill the boss.