I have to do that mission and the one where you go to the warehouse the detective points out on the map (it's not showing up for me, or I don't know where the supposed warehouse is). Have you already managed to get to the warehouse? If I unlock that, maybe I can help you with the cartel boss because that's the next step.
I figured it out after thinking about the game's logic.
You need to have all stats at level 14 (agility, strength, stamina) and be level 8 (by completing lots of nighttime events).
You have to make lots of round trips from the apartment rented from that weird guy in bunny slippers (that’s my advice because it’s faster) to the dungeon with the old samurai to practice all the combat methods until you reach level 14 in everything
Then go to the cartel’s mansion at night and do “enter,” “ambush,” “ambush,” “sneak up.”
And you’ll finally face the cartel boss
However, I still don’t understand what this “warehouse” mission you’re talking about is.