Three red barrels on the far side of the shielded door are the key - lay into all three with machinegun fire, and they will explode, destroying the mechanism keeping the shield in place. Now that you have Empower available to you, you can use it to shoot through shields, including the one shielding the doorway at the top end of the room. That second Tome of Power marks the final valuable you can find here, so with it in hand, backtrack to where the boss melee just took place. Shoot it out and grab the Tome of Power from the small crevasse it was hiding (2/2). On the right side of this new, yet-explored isolated room, you'll find a red-glowing tile on the wall if you're using Veil. When all is quiet, scour their bodies for needed ammunition, and then head through either door they came through. The shields they set up should be of no consequence to you if you use your new skill.
First, turn your attention on the enemies that stream out of both previously-locked doors on the far side of the room. Activate the switch and grab the crystal from the holder, which will give you the Empower ability, giving your firearms more power, and more importantly, giving you the ability to shoot through shields. Once the beast is slain, you can head to the control panel near the far end of the room. Each pillar takes two hits, meaning you'll need to reproduce this technique eight times for the portal to fall, and with it, the monster it most recently created. Then, stand between him and one of the yet-destroyed pillars, letting your foe do the rest of the work. So, while dodging the enemy at all costs, shoot at him just enough to get his attention. However, for the most part, you can holster your weapon, because the idea here isn't to eliminate the enemy, but rather to use him to destroy the four pillars on the main floor that are holding up the portal. A few Nazi soldiers will show up at this point, but in fact, your enemy is identical. After turning his ire on the officer, the monster will turn his attention to you. What comes out of the portal is anything but human, however. You'll witness a cutscene where a Nazi officer throws a helpless Nazi scientist into a portal the enemy has been working on.
After grabbing that, wrap around to the corridor ahead and head leftward to the double doors, where this mission's final enemy, a boss, as it were, can be found.
From there, look behind the door, and you'll find the final stash of Gold of the mission, worth $100 (23/23). A well-placed grenade supplemented by some gunfire will do them in quickly, even in their relatively well-protected position. As soon as you go through, expect to run into a final contingent of Nazi officers protecting their turf. The only way out of this room is on the far end of the area, via a door on your right.