The gate to the town is closed. Go left and shoot the SCAR gun up at the wooden walkway there. Blow it and climb up the stone wall. Drop down on the other side.
Bounty! On the left wall immediately after dropping is a wood door. Blast it open with the SCAR gun and you'll find Bounty Card 21 on the table.
Step back outside and prepare for a firefight. You're looking at two drones -- one on the ground, one on the second floor of a building ahead -- and a spider to the second drone's left. Another drone is nearby and will come to that second-floor area after you kill the others, more than likely.
Bounty! Look along the left wall here. You'll see a wooden structure that you can destroy. Underneath is Bounty Card 1.
Keep an eye out for additional enemies. There's a drone up on the roof above the area you just cleared, and another will appear there from the left alleyway.
When you're done with them, head up and use the nanomed machine at the top of the stairs. Turn left and head down the tunnel.
At the other end, you'll enter a large square with a tunnel to the right. Go up the stairs to deal with a new enemy: the Alpha Drone.
Note: You can also approach this area from the tunnel to the right. This affords you a somewhat decent place to fall back to, should you need it. Which you won't.
Quickly plug the drone with a SCAR round or two and doubleback down the stairs to avoid its big machine gun. When you can take a shot, put a few more SCARs in it. Four or five should do it. Detonate all of them at once.
Now approach the drone from the back and hit Y to execute it.
Achievement Alert! You'll earn the Kill an Alpha Drone Achievement for winning this fight.
Hit Y again while you're standing over the Alpha Drone to take a live grenade off its body. Quickly toss it into the doorway from which the drone first appeared.