How to beat Doom II level 30, Icon of Sin:

Short answer:
From the central pillar, fire 3 rockets straight through
the hole in the demon-faced wall.  You'll have to fire
before the pillar rises all the way to the top.  Try firing
when the top edge of your launcher lines up with the bottom
of the wall-demon's chin.
This will usually take a minimum of 3 separate trips on the
pillar to accomplish.

(Note: For a video demonstration of the walkthrough below,
click here.)

Detailed walkthrough:
You start the level at the south end of the map, in a small
chamber filled with power-ups, weapons and ammo.
The moment you move from your starting position, you'll
automatically pick up a backpack.  Now pick up everything
else in the room.

On some skill levels, you won't be able to pick up the last
few items.  Don't worry about it, as your ammo levels will
be at maximum anyway.

At the north end of the chamber is a teleporter booth; Once
you step into the booth, you can't come back.
Step in and you'll re-appear in a huge chamber to the north
of where you just were.  Ahead of you is a wide acid lake,
and beyond that, the game's Boss.  (See THE BOSS, below,
for more details.)

You'll be standing on the bottom step of three huge steps
that lead up, south, to a switch recessed into the middle
of the south wall.  Each of these "steps" is actually a
lift, or elevator.

Turn left (west) and run, following the step you're on,
around to the left.  You'll come to a demon-faced panel at
the corner on your left.  Activate this panel like a switch
and the next "step" up will drop down, then rise up again
in a few seconds.  Get on and ride up.

Now run east, to the far end of the step, and look for
another demon-faced panel at the corner on your right.
Activate it and get on the next step up when it drops.
Run west again, to the small, recessed room on your left.
At the back wall of this room is a switch.  Activate it,
and a very tall, four-sided pillar will begin to slowly
rise up out of the acid lake below, to the north.

From here on, you can ignore the switch you just activated;
it will no longer have any effect on the pillar.
Now prepare to go down into the acid lake...

Once the pillar has risen to its highest point, you can
touch-activate the pillar like a switch, and it will drop
back down.  After a brief pause it will rise again.

Get on and ride it up.  Get your rocket launcher ready, and
turn towards the hole in the demon-faced wall to the north.
You'll have to fire before the pillar rises to its highest
point, and your aim has to be nearly perfect along a flat
trajectory that goes straight through the hole.

Common advice is to wait until the top edge of the forward
end of your rocket launcher is exactly lined up with the
bottom edge of the wall-demon's chin, and press Fire.  You
may have to adjust this timing depending on your reflexes,
and the game's speed on your computer.

As the pillar is rising you'll only have time to get one
rocket through the hole, meaning that you'll need to drop
back down into the acid lake, Activate the pillar again to
lower it, then get on, ride up, and fire again.
Expert (or lucky) players may also be able to fire a rocket
through the hole as they are dropping down off the pillar.

It will usually take 3 shots straight through the hole in
order to kill the Boss, and you must use a rocket launcher,
as no other weapon will do the job here.

Between the time when you Activate the central pillar to
make it drop, and the time when you have to fire a rocket
at the Boss, you'll have a few seconds to attack the
monsters who will be appearing on the steps to the south,
and in the acid lake around you.  A BFG 9000 is a good
choice for this, but if you're attacking monsters from the
pillar while it's rising, be sure to reserve some time to
allow you to switch back to your rocket launcher and align
it for a shot at the Boss.

Note that there are a few power-ups around the acid-lake
area, including Soul Spheres ("Supercharges") on the steps,
and other power-ups on a raised area below the face on the
wall.  (You can run and land on this raised platform from
the top of the pillar when it has risen far enough.)
On skill levels 1 and 2, there will also be Invulnerability
Spheres around the base of the pillar.

If at any time you wish to get back on the steps from the
acid lake, simply walk up to the bottom step and Activate
any part of it like a switch.  It will drop and you can get
on before it rises again.

PC Doom 2 players can access a demo that shows a basic
victory at Ultra-Violence Skill, by clicking here.

THE BOSS
--------
When you appear in the large, northern chamber, you'll be
facing north towards a giant "demon-faced wall", the game's
final "Boss".  Destroy it and you win the game.
In the wall, in the demon's forehead, you'll see a small
rectangular hole.  From this hole the Boss will begin
launching an infinite number of small, skull-faced boxes.
Each box, at some point in its flight, will spontaneously
turn into a monster.
WARNING: If one of these flying boxes "spawns" a monster at
(or above/below) the player's coordinates, the player is
destroyed.  This is true even if the player is "invincible"
through the use of power-ups or cheat codes.

Good luck!

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