Mega Man Zero 2/Power Room: Phoenix Magnion

For this particular mission, Elpizo is acting on a rumor that an arms factory is situated close to the Resistance Base. His scouts found reactors that give it a steady supply of electricity, and if Zero were to destroy them, it would reduce the amount of troops Neo Arcadia would have at the ready.

Unlike the other missions you can pick, this one isn't set in a linear path. In order to complete it, you need to find four reactors and destroy them. The last one you destroy will trigger the boss to appear, and also affect the difficulty of the battle.

If you succeed with this mission, you'll discover that every single type of Pantheon in all of the first four mission areas will be eliminated, permanently. This will make a lot of the missions far easier, but hinder your chances of higher kill rates in the levels.

There are actually hidden side areas you can enter, and a higher-than-normal time limit to complete this mission and get full points, but what you need to focus on is the fastest and safest path to get there so you can come back. You'll find the Sub Tank from the Forest of Dysis is your best friend here.