Directional mines Carl planted along the road, keyed to detonate only when hunters pass over them.
On the drive south toward the river, Carl stopped a few times to plant claymore-like mines along the main road, configured so they would only be triggered by Hunters. He suspected he was wasting his time, but reasoned it couldn't hurt. The road was otherwise empty of mobs, at least near the main route, with wildlife and foliage of all sorts that the party had no time to examine.
Claymore-like directional mines buried along the main road.
Carl's roadside attrition against hunters funneling toward the river crossings.