The GL-YA area is, I feel, the key to this level. When you control this area,
you can keep the YA away from your opponent as well as have access to 15 rocket
ammo. This area has plenty of health packs and has very limited entrances, so
it's unlikely they'll be able to surprise you. Additionally, you can teleport
to the other side of the level instantaneously with the teleport below the YA.
When you have control of this area, you also basically have control of the quad.
You will have the better position if you happen to run into the enemy when you're
heading towards quad. There are two main entrances to the GL-YA area. One is
the hallway from quad and the other is from the YA side, either through the spike
traps or underneath them from the water.
GL/YA - The Quad Entrance
When you have control of the area, you generally want to walk out this door to
start heading towards the other side of the level. The quad room has several
spots that your opponent can be hiding in just waiting for you to walk by.
However, if, as you're walking into the quad room, you get tagged with a rocket,
you can just back up into the GL room, where you can get healed and also
re-armored. It's virtually impossible for them to chase you into the GL room.
If you're hurt, start backing up towards the YA room -- shoot at the ground of
the entrance into the GL room. If he walks in, he will need, at the very least,
enough health and armor to take a rocket. Oftentimes, if the person gets hit by
the rocket, he'll be bounced a good ways, which should buy you enough of time to
get around the corner into the YA room. In the YA room, there is a 25 health next
to the YA, as well as one below, near the teleporter. When you've gotten the YA
and back to a decent health level, you can go back into the GL room and finish off
your opponent, who is likely armor-less and low on health.
Situation #1
If he chose to hide around a corner (near GL or near the rocket pack), then
it's even better. When you come out of the YA room, you always need to be
alert. Check all the hiding spots before walking across the room. If you
catch him hiding there, he'll have little or no armor and decent health.
With you 150% armor and 100% health, you can afford to trade rockets with him.
He can't.

Be aware of the GL room
Situation #2
If he ran away, chase him. Chances are, he ran towards the 100% health area.
There are many big health packs there… that, combined with the attraction of
the 100% health, will persuade him into going in there to regroup rather than
near the green armor. Additionally, if you didn't hear the door open (to
green armor area), you'll know for sure he ran into the 100% health room.
At this time, you should have 150% armor, 100% health, and plenty of rockets.
He, on the other hand, will have at most 200% health, no armor, and possibly
low on rockets. Now you've got the upper hand.
Next:
Enter the Yellow Armor