So after reading all of these solutions I figured I'd throw one up.
I did this on normal - to get the infinite launcher which makes several other achievements much easier to get later on and I believe it's necessary for the Starsenal achievement as well.
I followed the IGN guide I found on youtube which I'll embed here at the bottom of this solution. I set my tablet up on my coffee table and literally watched it while I played the game. It made this achievement a lot easier than I initially thought. The video completes it in 2hr 43 minutes, I finished right at the 2hr 30 minute mark. I had to re-start a couple of areas due to deaths or just completely messing up and honestly I was stressing the entire time worried I wouldn't come in under the 3 hour mark - turns out even with several mistakes i still had PLENTY of time.
Some general tips -
Don't pause - timer still runs
Play as Jill!
SAVE! - i saved 8 or 9 times in my play through, it doesn't take long and if you felt like you've made good time since your last save, there's nothing wrong with saving and if you mess up on the next section you can go back and reload, I did this a few times. (especially on the puzzle box) You do NOT need to skip saving.
Don't shoot Yawn on the first fight, run right by him and just get the death mask and run back
don't worry about the blue and yellow gems - don't waste time with that statue
When you fight the dog for the collar, don't worry about killing the 2nd one - 2 shotgun shots first one is dead - i hit the herb box, got the collar and ran out
When you fight plant 42 - USE INCENDIARY ROUNDS IN THE GRENADE LAUNCHER! I followed the guide and he used regular rounds, I died like 3 times. I took incendiary rounds and I think it only took 2-3 shots. It was way easier
You can skip black tiger - I just ran around got the knife, cut the door open and moved on. There's herbs on the other side.
Electrocute neptune by grabbing at the key triggering the cutscene, push the generator into the water, then flip the switch
Let Barry die
When you get to the lab, you should only have about 20 minutes left in the game, maybe 30. In the video I link the guy took the time to burn every body on the b2f - you DO NOT need to do this. I didn't burn them and they hadn't turned into crimson heads after I killed Tyrant. You should be going fast enough to just kill them. I did not kill the one on the stairs - he is a puker and easily avoided.
While the same can be said for the zombies on the b3f - you should still burn them just in case unless you're really short on time.
When you get into the lab - the guy in the video really screws up the order you should do. This is my order and it shaved about 5-10 minutes off
Get into lab and save, grab lighter/kerosene/shotgun/grenade launcher
Kill 1st zombie in b2f - run downstairs ignore puker
turn left in b3f, shoot zombie/burn him. Go to end of the hall and go into the NE room - double doors. Run to the computer and enter in JOHN - ADA unlock b2f with CELL, unlock b3f and leave.
Go back upstairs to B2F run past puker, and go to the other room you haven't been in yet and kill the zombie on the way
go to the control panel on the right side of the room, enter 8 4 6 2 - go into the room it unlocks and grab the lab key. You can get the first aid spray if you have room/need it
Go back downstairs avoiding puker again. Go to the SE corner of the square and unlock both doors and discard the key - go into the east room.
Kill both zombies in this room, go into the south room. When you come into this room you can take the time to kill a ton of chimeras. I just went up to the capsule, killed the chimera there, got the capsule, and left. Go into the east room - it's the save room. I saved, dropped off the lighter/kerosene and booked it to the room at the base of the stairs in b3f, it's the northwest room of the square. Then go into the fuel fill room - kill the zombie, fill the fuel, then walk all the way back to the capsule and never ran into any problems.
From here to the end I just ran by every single Chimera and the only person I shot was tyrant - with Barry's magnum that i picked up from the item chest OTW to tyrant.
DO NOT FORGET THE LAB KEY WESKER DROPS - YOU CAN'T GET ON THE ELEVATOR WITHOUT IT!