To attain this achievement, you need to take no health (armor) damage, but shield damage is fine.
Difficulty does not matter, so the easiest method is to create a preset with enemy durability at 50% and the lowest fire rate, and to reduce your hitbox to the lowest setting. Play through stage 1 of the first mission, firing constantly, and you will pop the achievement no problem.
If you take health damage, restart the stage, as healing back your armor via upgrades or pickups to 100% by the stage's end will not count as having not taken damage.