Materials: enough money for the hell door and mystery box, 2 frag grenades.

This can work on either solo or online.

Open the hell door as soon as you have enough money for the three items above. From the doorway, throw a grenade at the mystery box then you need to quickly throw a grenade out of any window before the 1st grenade blows up. You need to run to the mystery box within 5 seconds after the grenade explodes then buy the box.

Finally, you need to spin 720 degrees before the weapon pops up and you get an instant ray gun.