This particular achievement can be fairly difficult. This needs to be done in survival and it’s much easier to do with a co-op partner, though I’m sure it’s quite possible solo. For this achievement you have to survive 15 waves without losing all your health and getting downed. Keep your distance from most enemies and spam your long range weapons or grenades. Make sure to heal with tequila whenever you’re at half health. On the boss round it’s easiest to use traps and shoot them from afar so you don’t take too much damage. Or if you have a co-op partner have him do most of the fighting so you can keep your health up. After round 15 you will get the achievement if you haven’t been downed.