【CAMPAIGN】Defeat Rachael before she gets to the cafeteria.
When you are searching for the Lift key you will head back to the area you were in at the beginning of the game to look for the girl you saw get attacked by Oozes (Rachael). She will be gone, but you'll now have the chance to chase her down and put lots (and lots) of bullets into her. One important thing to note, especially if you're trying this on your first playthrough, is that when she runs away, you can shoot her in the legs to slow her down. If you don't do this, and you don't have a magnum or at least some +damage heavy guns like a rifle or shotgun, then you most likely won't get her in time. She will officially "make it to the cafeteria" once you make it to the bottom of the two-tier staircase and fight her once more before she enters the vent.
On Infernal she has quite a bit more health, is harder to stun, and obviously hits harder. To counter this, I focused on upgrading my G36 (NG+) with the highest damage I had (80%), Auto-Reload, Crit +30% and most important of all; Edge Runner 2. To maximize your damage using this setup, let her hit you at least once to increase your damage output thanks to Edge Runner (up to +200% max damage if you take damage, but another hit from her can kill you). This method allows you to utilize a relatively common ammo-type as opposed to Magnum or Rifle Rounds.
Note: Always make sure you get her to spawn, and then run back to whatever door you came in so she has that much farther to run to you, and from you back to her next vent, allowing you ample time to deal plenty of damage to her.
This is tough but not impossible. I did this on 3DS but I used all my ammo doing it. What I would suggest is after earning this achievement reload your save and just run to conserve ammo to continue your playthrough. Everyone is saying just use the shotgun but for the record you will have an MP5 by this point as well.
Ch. 4 on Normal difficulty 5 grenades SMG-damage 1, stopping power 100% Shogun- none (might have used the one that increases damg. at close range Rifle- damage 2, load cap. 20%, Crit. & firing rate + 30%
Go up the stairs & kill the enemy on the landing with the shotgun then run all the way back to the door you just went through and wait in front of the door ( don't go through it). Dodge her attack then face the wall and throw a grenade. Unload some SMG bullets and throw 2 more grenades and it should knock her down. If she stops moving and opens her tounge , shoot it with the rifle. Switch weapons instead of reloading, and keep hitting her in the head. When she flees, try to beat her to the stairs ( I thought she got around me and got to the stairs b4 me). Use 1 more grenade and keep
Shootting her in the head. I got her in the hallway after bathroom. Snipe the hallway enemies, advance slowly in the bathroom for the ooze to stop peeing and reveal himself. I got careless and Racheal hit me b4 she escape. Summary 1. Shoot her in head 2. Kill her (if u can) or get her to run slowly, when she escapes the 1st room. 3. Switch weapons instead of reloading, use shotgun on 1st enemy and If u think u can't get her w/ rifle. 4. Make sure your ready and reload your weapons before u move on. 5. Shoot her when she is getting in vent
I just did this via running past her in the bathroom area (2nd encounter) and ran all the way to where you meet up with your partner. We then ran all the way back to where the bathroom was and she was still there (She hadn't spawned further on) this meant that having Parker to provide a bit extra fire power.Managed to kill her in the bathroom area.