Where to Grind Battlepower Character with Your Team?
In that Mang Mountain Pit, you need to turn down the offer of the Tomb Raiders and fight them. Then you need the machinery skill to open the door.
There’s respawning robots inside that grant combat points. If you accept the Tomb Raider offer the door permanently closes and you can’t do this. It remains open if you open it yourself and you can keep killing robots.
To get machinery skill you need to visit the Tomb Raider faction before you make them enemies and learn it from their leader.
The nightwatchmen that appear in towns I’ve read also can teach it but I haven’t tried it. The nightwatchmen are different from the Bailifs.