Method for starting a boss-monster fight in GBA Doom II
level 21: Gotcha!

This situation in GBA Doom II is similar to what happens on
the same map in PC Doom II.  One difference in the GBA game
is that all monsters in this version have a much shorter
line of sight, and a monster that has been alerted to a
threat can drop back into "wait" mode if their target moves
out of range.  This can make it a little harder to start a
fight between them without getting caught in the middle.

When you first enter the chamber with the Cyberdemon and
Spider Mastermind, try running down to the base of the
Spider's pedestal as soon as possible.  From there, hug the
side of the pedestal, peeking out just enough to get the
Cyberdemon's attention to draw its fire.

Try to stay close enough to the side of the pedestal so
that the Spider can't see you over the edge.  As the
Cyberdemon fires at you, try to maneuver to guide its aim
towards the Spider's pedestal.

With a little luck and practice, you can get the rockets to
hit the top and edges of the pedestal, hitting the Spider
with splash damage.  This causes the Spider to retaliate,
starting a fight between them.

Good luck!