To beat the pair of large monsters on level 20, Gotcha!:

On the ledge that runs along the perimeter of the room, go
around to the east side, and stand behind the Cyberdemon,
so that the Cyberdemon stands directly between you and the
Spider Mastermind on the far side.
(If the Cyberdemon has noticed you, be prepared to dodge
some rockets.)

What you're trying to do now is to get the Spider to shoot
at you, but accidentally hit the Cyberdemon with bullets.
When this happens, the Cyberdemon will return fire against
the Spider, and both will fight it out to the finish.
By nature, the Spider is weaker than the Cyberdemon, so the
Cyberdemon usually wins, at which point he will usually
resume his attack on you.

One strategy is to attack the Cyberdemon while he is
distracted by the Spider.  This helps even out the battle,
and makes the survivor as weak as possible, so you can
dispatch it quickly.
Note however, that when you attack either monster, its
attention can sometimes be redirected back to you.

Another strategy is to wait until the Spider dies, then
just fight the Cyberdemon, because it's easier to dodge the
Demon's rockets than it is to dodge the Spider's endless
hail of bullets.

You may also choose to reverse the above situation from the
beginning, by starting off behind the Spider, and waiting
for the Cyberdemon to hit it while shooting at you.
If you choose to do it this way, you can try keeping the
red pillar (on the edge of the Spider's platform) between
you and the Spider.  This can help keep you from getting
shot by the Spider while you wait for the monsters to start
fighting with each other.

Note that in this room is a Soul Sphere ("Supercharge") and
a rocket launcher.  There is also a secret chamber nearby,
where you can find some rockets.

PC Doom2 players can access a demo of this level that shows
one way to defeat the monsters, by clicking here.