The Acropolis - Skirmish Inside the Acropolis Tower (Part 2)

Once you've killed the first 13 regular soldiers, 2 armored enemies arrive on the scene. One has a metal shield.

If you can hit the one without the shield using a grenade arrow while they're still standing close together....

...you can probably kill them both with one shot.

Otherwise, you'll need to keep moving and try to split them up. Hit the shielded guy in the feet with a grenade arrow or two poison arrows to bring him down.

Boom! (Or, if you're out of special ammo, let him come to you and use the Dodge Counter we covered back in the Copper Mill walkthrough.)

Then take out the other guy with a grenade or several shotgun blasts. It will take that much to chip away at his armor and then finally kill him. When these two fall, the fight is over and a cutscene plays.