The crate must be placed by a large stone gate. Climb up and make a single jump (#1). Thanks to that, you will be able to enter the new corridor. You'll find yourself near some new traps. You will have to start avoiding them. Use the grapple to target a small crate (it's on your left). You will find it on the other side of the trap (#2). The main problem is that the traps will make it a lot harder to pull the crate. You will have to wait for the right moment. The blades are slowing down at some point. That's when you should start pulling the crate. Move it to the main area, because that's where you'll need it.
You should know exactly what has to be done right now. The newly acquired crate has to be placed under the trap (#1). Thanks to that, you should be able to block the mechanism. Make a dive roll in order to reach the other side of the corridor. Keep pulling the crate towards Lara. You will have to repeat this combination in a few seconds. You should be able to reach a fiery corridor very soon (#2). Just as before, place the crate on the first platform. Now you will only have to make two precise jumps.
You should be able to reach a new room very soon. Take a look at the pressure plate (#1). You will have to find a object that can be placed on it. Thanks to that, you will unlock a nearby gate. Start off by turning left. Head on to the water and start swimming towards the right platform. You can exit the water now (#2).
Turn around a make a few steps towards the water. There's a large chandelier here. Use the grapple and make a single pull (#1). The objective is to let it start swinging. As a result, the chandelier will destroy a part of the wall. Thanks to that, you'll be able to see a single coffin. Once again, you will have to use the grapple. Aim at the coffin. You will have to start pulling it towards Lara (#2). The coffin must land in the water.
Head on to the water. You will have to reach the top of the coffin. The objective is to transport it to a nearby pressure plate. How? It's very simple. :-) Use the grapple to aim at the highlighted lanterns (#1). You shouldn't have any problems with that, especially since there are a lot of them in the area. Place the coffin on the button and start running towards a new corridor (#2).
Once again, you will be moving on a top of the coffin. Head on to the lower platform. You'll find yourself in the water. Start off by using a nearby lever (#1). Bear in mind that most of them will be hidden under water. You'll unlock a nearby gate. Climb up to the top of the coffin. You won't have to do anything at the moment. The stream will help you to reach your destination. You will have to make a single jump along the way. The objective is to reach a small passageway (#2). Head on to the water.
You shouldn't have any major problems with finding a second lever (#1). Just as before, you will have to pull it in order to unlock a second part of the corridor. Climb up to the coffin. This time you will have an opportunity to attach the grapple to most of the nearby lanterns. There's a small passageway on your right (#2). Use it. Start moving forward. Don't worry... Lara will be just fine.
A short cut-scene will appear on your screen. Lara will find herself inside a HUGE cavern. Here's where you'll find the boss of this level, however you shouldn't be worried about that at this particular moment. Start swimming towards the main entrance (#1). Once you're there, it would be wise to take a closer look at the whole cavern. Now you will have to position two of the nearby horizontal poles. The left one should be left alone. You'll have to find the right one. Use the grapple to pull it towards Lara. The pole should be facing the water now (#2).
Turn left and start moving forward. There are some narrow ledges here. You will have to use them to get to the upper level (#1). Once you're there, make a jump towards the horizontal pole. Now you will have to reach a slipway. Be careful. You will have to make a jump at the end of the slipway (#2). As a result, you should be able to reach a pole. It will turn left (along with Lara).