How to get the blue Mega-Armor and second Soul Sphere in
Doom 64 level 15, Dark Entries:

First, go into the structure where the Mega-Armor and Soul
Sphere power-ups are.  At the far end of this structure is
a window.  Walk up to the center of the window and look
straight out.

In your field of view out the window you should see a pair
of switches near the ground: One ahead and to the left, one
ahead and to the right.
Select the chaingun, and quickly sweep bullets from left to
right across both switches in one slashing motion.  Be sure
to hit both switches, left one first), and very quickly Run
(remember your Run button) back to the power-ups.
(A pistol or shotgun will also activate the switches, but
you may find it easier to use your chaingun.)

These power-ups are each sitting on tall, narrow pedestals
which are surrounded by two sets of barriers.  When those
switches are activated, the barriers drop down momentarily,
then rise up again.
Before the barriers rise, you need to quickly run to stand
near the base of the exposed pedestals.  Ride the barriers
back up and simply grab the first power-up; Then repeat the
procedure with the switches for the other power-up.