How to you kill the creature with the hammer? And what does that Lever do?
You can't kill him.
The big wheel will open the gate in the wall on the right. You have to turn it long enough so you have enough time to get through.
But there will be a little problem. Behind it is a ladder wich you cannot climb up as long as the guy with the hammer is outside. So you either have to lock him in the chambers to gain some time... or you climb up the ladder without putting your feet onto it. If you stay on either side of the ladder Lara will only grab hold, but won't put her feet up. This way you can climb to the top without falling down.