There's the long platform in front of you, where you can find ammunition, health and armor. Gather everything you can, reload weapons, save the game and approach the door. It's time for the final duel with Makron (#221).
Stroggs' leader is a though enemy, but not immortal. The fight can be very easy or really hard - it depends on the Dark Matter Gun's ammunition that you have. Traditionally, Makron energy is showed on the belt located in the top part of the screen. You have to deprive him of energy twice (during the second approach he'll begin to fly). Hit Makron with all you got (Dark Matter Gun is the best option). Try to run all the time and avoid enemy's bullets. This is the revenge time!
Killing Makron doesn't end your adventure with Quake 4. As soon as the enemy falls, save the game and use one of the teleports (they are located in every corner of the vast hall), get to the balcony. Quickly recover energy by using machine placed on the wall and wait until the protected by force field brain will appear in the center of the room.
The organ isn't dangerous, but the enemies that will show up suddenly on the battlefield, are. Try to eliminate opponents and shoot with any machine weapon to blue bar (#222), located under the ceiling. In this way the brain will be deprived of the force field and than you can start frontal attack. Rocket Launcher is the best option here (it's fast and makes a lot of damage), alternatively use Railgun. The brain will again obtain the force field after a while, so repeat the whole operation. If other enemies are really bothering you, shoot in their direction with Dark Matter Gun.
After exhausting fight, the brain will be destroyed for good. Your adventure with newest Raven Software's game has came to an end. You can't do anything more, but only wait for possible expansion...