Proof for chief Briggs

[The Saint]

Ok, that may be really stupid problem, but I don't know how to begin the expedition. I wasn't able to persuade chief Briggs to allow me join his expedition, and now he wants proof of being hardcore. I killed almost everyone in Arena (except Carnifex), but in dialogue with Briggs he said it is not enough. I tried to impress him with finishing the Gauntlet or joining Oculus, but I still see no option to join him.

[Tamior]

Finishing Core City corporation quest line worked for me.

I've also heard that killing Balor is sufficient.

[The Saint]

Ok, killing Balor did a trick.

[Carfrei]

2 Ways to "proof" you're a worty soldier for the expedition are:
-Becoming the "Invictus"
-Almost complete one "olligarc" Quest lines(finish main part of tracking acid hunters"

[Azura_04]

2 Ways to "proof" you're a worty soldier for the expedition are:
-Becoming the "Invictus"
-Almost complete one "olligarc" Quest lines(finish main part of tracking acid hunters"

Also joining the Protectorate does the trick.

[RalleYTN]

Simply joining the protectorate does the trick because Briggs searches for soldiers and knows the commanding officer of the protectorate main base of south underrail personally.

I also started the expedition. I give you big tip because there are creatures that are annoying as fluff:
take a .44 Hammerer, .44 Explosive Ammo and armor that protects you against both bio and mechanical damage.
you will thank me.