Serious Sam 4

You will get this achievement upon the completion of the mission "Machina ex deus". You will end the mission fighting a massive boss, just keep shooting it in the face until it closes and then mob the area (Kill all enemies) and it will re-open for you to finish it off. To make this easier make sure you have infinite ammo turned on and even tourism difficulty or play on co-op and set respawn timer to 0.