I'll note that I seemed to have had an oddly easy time for this mission. There just weren't many super mutants around, but I thought there were when I played through it awhile ago. So...stay alert. You'll face some array of leveled super mutants. Super mutant overlords can spawn here, so bring some serious weaponry if you're above 20. I saw one at level 16 though.
You need to go over some part of the broken fence or head into the camp area. There's some medical supplies left, but you can feel free to ignore it. There are 2 possible points of entry into the police headquarters. There's a door on the ground floor that you can lockpick if you have 100 points in it. If you have enough points, then feel free to do it. You'll get to skip a few battles on the top floor but also miss a little loot. It's your call. The other door is a broken wall with an exposed stairwell. Climb up the stairs and go along the ledge to find the door to the top floor.
This is just an office area. There should only be 2 super mutants here, and they're actually having a conversation. You can swing around the door way and take out both of them. The hunting rifle is still effective, but the shotgun is naturally a bit better in these close quarters. Either way, use the office walls as cover and take them out. You should then be free to explore the area. Just be careful, since there are a lot of mines. They are a little hard to spot in the ruins, so look for the red light. They are also placed in strategic areas, so don't just run up to the first aid kits or the safes.