DOOM 3: BFG Edition
59 Achievements
50-60h
That was Close!
Kill an enemy with 1 health remaining in DOOM 3, RoE, or Lost Mission
9.9%
How to unlock the That was Close! achievement in DOOM 3: BFG Edition - Definitive Guide
While playing on Recruit difficulty, find a place where there is a lone Imp in a room with you. The Imp's fireball will do 3 points of damage to you, while their melee attack (claw swipe) will do 2 points of damage. Get in as close as you possibly can to them, and allow them to attack you. As long as you are within claw range, the only attack they will use will be their claws, thus taking you down 2 health points per attack. As long as your health ends in an odd number, they will eventually knock you down to 1 hp. At this point, simply retreat out of their reach, and dodge their fireballs while shooting them. Before long, they will die and the achievement will be yours.
If your health ends in an even number while doing this, briefly duck out the Imp's melee range (back up 2-3 steps), and allow them to hit you with a single fireball. This will drop your health just enough so that it now ends in an odd number, and you can proceed as noted above.
If it helps, when I did this I was on the Enpro level, in the room immediately after finding the Plasma Rifle in the locked cabinet in the office. This was also when I realized the amount of hp they were doing with each attack, so it could have been done much earlier in the game.
Note: The trophy description for this is misleading as it's you that has to have 1 health remaining, not the enemy.
This is easiest done on Recruit as a lot of enemies will only inflict 1 HP damage to you (such as Z-Sec Pistol guys). Just wait until an enemy drops your health to one, then kill him. It's likely you'll get this trophy naturally as you play the game. The Imps take 3 HP off of you so if you're at 100 health, just keep letting it hurt you until you reach 1 HP.
The best way to do this would be to let a weaker enemy (such as a zombie or Z-Sec) get your health down until you reach 1 HP, then kill them before they can kill you. On Recruit, pistol Z-Secs only take 1 HP each shot at close range. You could even try hurting yourself with barrels or grenades to aid in the process.