The fight with final boss is divided in three parts. Start running around the huge guard and try to keep the safe distance, because otherwise Lara can be easily caught by him. Fortunately this doesn't mean the sudden death, but the main heroine will be seriously injured.
Equally important thing is to avoid boss standard attacks performed with his huge club. Take some time to familiarize with those attacks - ground hits are very predictable but club swings are more dangerous. Of course you should help yourself with dodge key/button, trying to remain in motion all the time.
Do not try to blindly shoot at your opponent, because he is heavily armored. The only soft spot on his body is hi back, so you should decide to shoot him after his failed attack. During the first phase of the fight it is best to use the shotgun, because there are no additional enemies on the arena.
Keep attacking boss's back until you see an icon over his head, meaning that you can perform a melee attack (screen). Quickly run towards the guard and press appropriate key/button. After a moment you will have to use it again, so do miss that point (if you make a mistake, the guard will recover a bit and the fight will be longer).
The second part of the fight has one major difficulty - new archers will be regularly appearing on the arena. Kill only those, who clearly stand on your way (the rest will die as a result of boss's attacks). I recommend using the shotgun and not risking the close combat because in the meantime Lara could be hit by boss. It is also important to examine archers' bodies when possible, because they leave ammo.
Attacking the huge guards doesn't differ from the previous phase, so try to avoid his attacks and look for the chance to shoot his back. You should try to shorten this fight to the maximum, so I suggest using the rifle instead of the shotgun.
Fight until a well-known icon will be shown on boss's head. Run towards the guard and press the melee fight key/button in appropriate moment.
The last, third part of the fight is connected with dealing with archers attacking Lara. Act the same as before, so kill only those enemies, who clearly threaten the main heroine.
In turn, the big simplification is fact that you do not have to shoot only boss's back, because he lost his headgear in the meantime. So shoot at his head now (also in this case it is best to use the assault rifle).
Wait until the execution icon appears over guard's head again. Attack the boss and after a while press the melee fight key/button again.
In order to finally eliminate the huge guard, you have to rhythmically press melee fight key/button and then interaction key/button, after appropriate icons appear on the screen (reward: 100 XP).
Use the newly unlocked passage and head towards the pulsating sphere, inside which the ritual takes place (reward: 100 XP).
At the end prepare to fight Mathias - fortunately the entire fight is only not very complex QTE sequence. Start with shooting the gun and then press the melee fight key/button at the right time (screen).
Soon you'll be forced to rhythmically press interaction key/button. When Lara obtains two guns, start pressing aiming and shooting keys/buttons alternately.
Now just enjoy the end of the game - congratulations! ( Note - if you haven't finish the game in 100%, then after returning to the main menu select Continue option and load the last save. You'll be moved to the island and you'll be able to complete all optional tasks.