The Rachael boss fight and the key to the lift concludes Chapter 4 of Resident Evil Revelations, a tricky encounter as she stalks you through the ship.
Rachael boss fight
Once you're ready to begin the battle, leave the room and make your way back up the stairs. A mutant will appear from around the corner, closely followed by Rachael - deal with the mutant quickly so that you can focus your full attention on Rachael when she arrives. Your task here is simply to keep inflicting damage until she runs off and slithers into an air vent.
Rachael can be a little tricky to predict though, as her tripping, stumbling style of movement causes her to either slow down or stagger forward at increased speed. You'll want to keep backing away from her as you shoot, maintaining a safe distance (which can be difficult in such claustrophobic confines), and be ever-prepared to dodge when she gears up to sprint forward and grab you. After a successful dodge, put some distance between you both then immediately about-face and resume fire.
Deploy your most powerful weapons, such as shotgun or machine gun, to shorten the length of the fight, and take advantage of the moments when she pauses to laugh maniacally - you'll have ample opportunity to unload bullets into her brain here.
Note that dying during this sequence rather tediously returns you to the bloodstained starting room, and you'll need to take Rachael's diary again (and sit through the proceeding cutscene) to get the boss fight going once more.
Once Rachael flees back up the stairs outside the starting room, chase after her, firing a few shots at her back, until she leaps into the vent. Start your journey back to the elevator, and be prepared to encounter more mutants on the way - these are placed randomly and will change on each new attempt. When you reach the far end of the shower room, Rachael will spawn behind you to the west. Immediately about-face and resume battle.
After she escapes again, continue toward the lift. Don't forget to scan your surroundings and hoover up any likely-much-needed green herbs and ammo. Back out in the corridor, Rachael will strike again as you turn the corner so be ready to open fire then proceed forward once she flees. Continue forward and prepare for yet another ambush as you exit the stairwell and reach the junction in the corridor outside - Rachael will plunge from an overhead vent. Here, you'll immediately be pinned against the door so send her staggering back with as many shots as you can muster, then dodge out into the wider corridor for a little more space.
As soon as she scarpers, proceed through the kitchen (which hides more handy ammo) and into the cafeteria. As you near the purple vending machine, Rachael will again drop down from above and attack. This marks the last part of the battle against Rachael, and is also the toughest segment, as a large number of will mutants continuously spawn alongside your main target. Thankfully, Parker puts in an appearance now and provides some assistance. As such, focus your attention on Rachael and leave the mutants to Parker, unless they get excessively close.
This final section of the fight begins as soon as you near the pipe hanging over the vending machine. A good strategy is to inch slowly toward it then, as soon as the first mutant plunges into the area, start lobbing grenades. You'll get through the grenades fast, but you'll clear out a bunch of mutants AND do some preliminary damage to Rachael this way.
Get the key to the lift
Your best tactic for the rest of the fight is to position yourself at one end of the U-shaped path that forms the room and aim shots at Rachael as she ambles forward. As she closes in, duck under her, head to the far end of the path and repeat. Eventually, she'll go down for good, and your reward is the LIFT KEY. Finish off any mutants in the area then claim your prize.
Ride the lift down to the Bilge room and retrace your steps to the elevator platform in the next area. With the key in your possession, you can finally start the machine. Interact with the control panel in the centre of the elevator platform then yank the lever on the right. Your long-awaited descent will finally begin, bringing a start to Episode 5: Secrets uncovered.