Easy Adv. Power armor in F2:
make sure you buff your sneak skill for this mission. I had mine about %35, or %40.
You can get A.P.A. right after you leave Arroyo. The first step is to travel down to San Francisco, directly south. Enter, and find the Brotherhood of Steel Compound. Talk to the guard outside, and he tells you of a place called Navarro. Agree to get the Vertibird plans for him, as well.
Now, go to Navarro. It should be north. When you get there, there will be a man in a purple robe standing alone. I believe you can either ask him to join the Enclave, or simply kill him, to gain access to the shed behind him. Inside is a trapdoor. Go in.
This is where your sneak skill comes into play. Head down the corridor, open the door, and SNEAK past the guard. Otherwise, his plasma rifle will probably liquify you right off. Head to the room filled with lockers, across the hall. Inside of one of those is the Adv. Power Armor.
Once you wear the armor, you can roam freely about the base. Head up the elevator to ground level, and grab the Vertibird plans by talking to a few NPC's. Return to the tunnel, and head back to San Fran.
Once you get into the BoS compound, you get lots of neat goodies, including some old-time power armor. Sell and/or equip at will.