Completely without sounding any alarm, around 1 minutes to complete on the highest difficulty? With a little practice it’s actually possible.
First, you need to be four people. It might work with 3 that have done this often enough but it’ll be much more simple with four at an earlier level. The main objective is to get in, steal jewelry and get out with no cops showing up.
You start by looking for the Jewelry store mission, on overkill. You can tell it’s on overkill if it has 3 yellow dots underneath the name of the mission. It’s a low profit mission so it will always show one white dot.
You want all of your team to go inside the store. If there is a guard in there, have one with a silenced weapon ready to take him out as soon as masks goes on. Remember to answer the pager when you kill a security guard. When you mask on, have one team mate put down an ECM jammer. This will stop civilians from calling the cops right away.
Then it’s time to smash and grab some jewels. Have two people inside going through the display cases and bagging the jewels, throwing them to the front of the store. On the outside, have two people running back and forth between the store and the escape car, throwing bags in the back and running back for more. You need a total of 8 bags and it should go pretty quickly. Those on the inside should focus on getting the “baggable” jewels and not the smaller single items just so those on the outside won’t have to stop and wait for bags.
After a while you’ll get a sense of how much you can get away with but remember that you only need eight bags and then you’ll get a ton of experience for it. (see video below for an example of the run)
This will work great up until you get around level 30 and then you might want to consider doing Nightclub on overkill, which I’ll write a little about later.