The sixth stage is known as the Energy Zone. It will be considerably difficult to complete if you attempt to rush through it, so you are advised to take your time while getting through it. Run to the right and shoot the soldiers that'll come down from the step-like ledges ahead. Kill the sniper waiting near the weapon sensor ahead. The sensor will contain a Machine gun. There'll be a Gunner on the ledge on the far right. Kill him and drop down to the very bottom to kill the Gunner waiting there. Yet another Gunner will appear once you reach the break in the middle ledge. After removing him, continue right until you see a large gap, above which is a pipe with a small flame. There is a Laser gun in the nearby weapon sensor.
The flame from the pipe will shoot out as soon as you try to pass by it. To get past the flame, you'll need to jump towards it so that it shoots out and then quickly jump across as the flame is retreating back to the pipe. If you happen to die trying, jump across while you're temporarily invincible. On the other side, there will be two more Gunners waiting for you. Kill them and prepare for another flame gauntlet ahead. Be careful as you head to the right because there will be another flame here, this time coming from the wall on the far right. You'll have some warning as to when to crouch, but do so quickly or you'll be killed. Wait for the flame to retreat then quickly head closer to the wall to the right.
After the flame retracts, get ready to jump to the middle ledge when the middle flame dies down. Then jump to the top after the top flame disappears. Once you're at the top, fall down to the bottom on the other side and repeat the process. When you're back at the top, kill the Sniper on the opposite ledge. Then inch forward until the flame above starts shooting out. After it retreats, quickly jump to the other side. The weapon sensor below contains a Barrier item, but it's very difficult to collect without dying unless you shoot at it first. There's another flame that shoots out from the wall on the far right. Inch forward again and then crouch when the flame shoots out, then move toward the slightly lower ledge near the wall.
Once you reach the wall at the top, wait for the flame at the top and the middle ledge to retreat and then drop down. Do the same thing to get to the bottom. Note that you'll stand up momentarily when you drop down so be careful. Kill the two Gunners ahead and then jump to the middle ledge. Kill the Gunner ahead, jump to the top ledge, and then drop back down to the bottom when you're at the edge. Do not try to fall from the side of the top ledge because you'll be struck by the flame just ahead. It extends nearly to the bottom. Once you get past the flame, the boss is just ahead.
Boss: Armored Giant
The typical pattern of this boss is to slowly run to the left and then back to the right, at which point he'll throw out small spinning discs that move along the ground. The best way to avoid the discs is to jump over the first two, and then leap over the third one if he happens to throw it. He'll usually pause briefly because he throws the last one so that you have enough time to react. Wait on the far left and shoot at him, then jump over him whenever he gets too close. He'll start changing colors to reflect the amount of damage you've done to him. Eventually, he'll blow up after turning orange/red.