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Once the path is clear, advance into the Union controlled field. You should catch a glimpse of an agent killing a sharpshooter. That's what we call foreshadowing. Just walk past all of the soldiers, since you're not going to take part in their battle. Welsh is on a rock in the background. You need to approach him with a fully loaded Henry rifle. Once you talk to him, about 12 agents will phase into the battlefield.
You need to look at the one spawning right next to Dexter. Use the Henry rifle to nail him in the head twice as soon as the portal dissipates. Once he crumples, grab his rifle. It should be a nice M7 rifle. Use this to wear away at the advancing agents. You should have a good bit of time before they make it across the field. Use the scope to pick off a few, then hide to heal. Be very careful, since their automatic fire can kill you quickly.
The only real danger is that one can sneak up to the left side of the rocks. Dexter should be able to keep you covered, but you need to pay attention. If a few of them make it up here, fall back to the tree by the back if the camp. Once you've taken out the bulk of them, you should be able to retreat with Welsh to the marked exit. Jump inside to get back to KronoteK.