After eliminating the guard, nothing happens; it only tells me there's one guard left. When I eliminate the guard, the same thing happens again.I hope you can tell me a solution to the problem quickly. 
You must complete the abandoned cinema mission and have reached level 8, then You need to have all stats at level 14 (agility, strength, stamina) (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