He keeps saying not to kill the boss, so what are those numbers for?

The method requires getting very close to killing the boss, but NOT killing him (with shots). Once he's down to under 1000 points, the Titans commit suicide, and the Hunter gets really close and kills himself with Dragon's Breath. He dies, the boss dies a few seconds later from the flames. Loot drops, but the game respawns you as though you failed.

If I understand correctly, here's how you do it:
1- burn the boss down
2- everyone but 1 kill themselves
3- the last guardian alive runs up to the boss & fire dragons breath at their feet (and the bosses feet)
4- the last guardian dies
5- the boss dies during the "darkness consumed you" countdown timer,
6- boss drops loot
7- countdown timer finishes & everyone picks up the loots & starts over at #1.

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Hit me up Cruel, I'll be around.