Tatsuo is the enemy Agent you must face at the end of Chapter 4 - A train to catch. I never had any problems with him but he apparently has propensity to glitch halfway through the fight and simply never comes back down after you clean up the ankle biters. Resetting checkpoint here does not invalidate the achievement as long as you still hadn't used UAVs beforehand.
Make certain you're playing on at least normal as the achievement text suggests. The only thing you are not allowed to do during this fight is restock ammunition from the UAVs floating around overhead, so be careful not to accidentally hit
long enough to hack one. You can, however, pick up guns and/or restock ammo from as many firearms lying around on the ground as you wish. The entire purpose of the waves of enemies before Tatsuo appears (and the wave that assaults you when he loses half of his health) is to provide you ammunition to take on this boss without using the UAVs.
When you play levels from chapter select, you have no chip upgrades and the game will give you as many unallocated points as you would normally have spent by that point in the campaign (in this case, 2 points). My recommendation is to spend these on Protective Shielding and Synchronize, which are at the bottom left corner. Your weapon loadout by default includes the TAR-21 assault rifle and the Gauss rifle (the gun that locks onto its targets and fires homing bullets). If you prefer, you can simply hold onto the Gauss rifle and save that for the boss fight, but given the amount of ammo lying around from dead soldiers, you are guaranteed to get this achievement if you do not carelessly spray your bullets around.
For those not aware, the DART overlay (the orangeness that occurs when you hit
) will slow down time, decrease damage done to you, and increase damage you deal while it is active. This is something I had not figured out this early in the campaign, so use DART mode as often as possible when firing at Tatsuo and you should gain this easily.