The final level starts right from the start with a battle. You'll have to fight the Big Fat Ugly Guy.
Although he's pretty much invincible, he's really easy to kill.
How to defeat the Big Fat Ugly Guy
- Shoot him until he becomes angry and comes to charge you
- Position yourself near the edge
- Hold the RIGHT-click and press SHIFT + Left/Right - Lara will dodge him (make sure you DON'T shoot)
- When the red target is locked, shoot
- The Fat guy will fall over the edge, holding with just one hand
- Shoot the hand
- Repeat this couple of times
First, one arm will be destroyed and then the other, until eventually he falls into the lava pit.
Trust me, he's easy to kill.