Brutal difficulty will unlock once you have finished playing the game on normal difficulty.
The only difference when playing on brutal difficulty is that you die quicker and enemies take longer to kill. Here are a few tips to help you get through it:
* Use your glaive whenever you can, but do not rely on it. Use your guns whenever the situation demands it.
* Always use cover. Being out in the open while fighting will guarantee a quick death.
* Always aim for the head. Especially with those annoying monsters that jump around and fire coloured bolts at you. A headshot (with a glaive or gun) is the only thing that will kill them in one shot.
* Save all of your cash. Then buy the Korbov TK6 when it becomes available in chapter 7. Equip it with the following upgrades - Accuracy, Enferon Shells, Fire Power, Puncture. This will be enough to take down most enemies with just a few bullets. Equip your Tekna 9mm with the following upgrades - Fire Power, Accuracy, Puncture.
* Whenever you come across a section in which you can use a mounted machine gun, DON'T use it because i) you leave yourself open to attack from behind, ii) the gun itself is useless at killing things. Just hang back and pick off your enemies with your glaive or the Korbov TK6 if you have it.
* If you want to conserve your ammo, pick up enemy guns. You won't have long before it explodes, but in that time you can spray as many bullets as you like because the enemy guns don't use your ammo supply.
NOTE: There is a bug that sometimes reverts the game back to normal difficulty around chapter 5. It doesn't always happen, but I recommend that whenever you start a new chapter, quit to the menu, go to load game and make sure that the setting is still on brutal. If it is, go back and click continue. If it ISN'T then you will have to replay the previous chapter.