You can cheat, or here's part of doom2 faq by Tim Brastow at gamefaqs.
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Level 30: Icon of Sin
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Congratulations, you have made it to the final level of Doom II! Anyways, when you start this level, you start standing on a Backpack. Collect all of the weapons in this room, all of the ammo, and the Megasphere. Now go through the teleporter in this room. When you teleport, the final boss of the game is across from you (it's a large demon head on the wall).
The final boss also shoots out skull headed boxes from it's brain, containing an enemy (it won't be a Zombieman, Shotgun Guy, Heavy Weapon Dude, Lost Soul, Spider Mastermind, or Cyberdemon).
Also, go left, and then at the end of this platform, go left again. You should see a Demon Head figure on the wall. Press the spacebar on it, and the platform will lower. On this platform, go all the way straight, and you should see another Demon Head figure on the wall. Press the spacebar on this figure and the platform will lower. On this platform, wait for it to raise to the top. At the top, go through the only opening up here. In this opening, hit the switch.
Now turn around. You should see a pillar rising. Wait for it to get as high as it can go, and try to kill any enemies distracting you. Once the pillar gets as high as it goes, go all the way straight, to the pillar, and press the Spacebar to lower it. When it gets to the ground, get on it, and equip your Rocket Launcher (it's the only weapon that can defeat the final boss). Now, facing the final boss, wait for the pillar to raise again.
See the final boss's brain? As the pillar is raising, time your Rockets just right so it can get through the brain (you'll hear a loud scream). Weather you get it through or miss, get to the ground, lower the pillar again, and keep firing your Rockets into his brain. Repeat this twice more to win (3 hits in total).作者: pillgreen 時間: 2004-11-3 02:02 AM
, or here's part of doom2 faq by Tim Brastow at gamefaqs.