"Kill 25 enemies as they rappel using the Nagant sniper rifle. Unlocks the Constable mask, the Silenced Barrel and the Long Barrel for the Nagant sniper rifle, “Gunsmoke” material and “Dazzle” pattern."

This one asks for you to kill 25 enemies while they are rappeling down to the ground with the Nagant. Pretty simple.

Like most of these Achievements, you can do these pretty easily on Hoxton Breakout. Good xp, good cash. Can both days.

"Kill 25 enemies while in stealth using the Trench knife. Unlocks the Red Bear mask, Short Barrel, Suppressed Barrel, Short Mag, Folded Stock, No Stock, Solid Stock, Heatsinked Suppressed Barrel for the Patchett L2A1 submachine gun, Barrel Sight 44, Damper.L 44 Nozzle for the Broomstick pistol, “Red Sun” material and “Death Dealer” pattern."
This is one of the first achievements I got. It's plain as day. You can grind stealth missions like Shadow Raid, the bank heists (cash, gold, deposit, GO Bank, doesn't matter) with other people. Once you do that, you are able to do this next achievement.

"Equip a Broomstick pistol with the Damper.L 44 Nozzle and the Barrel Sight 44. Unlocks the British Bulldog mask, High Capacity Mag and Holster Stock for the Broomstick pistol, "Army Green" material and "Filthy Thirteen" pattern."
Yep, not only is this a Star Wars Reference, you actually get to make the first Star Wars Gun. Since y'know, the props in the original Star Wars were modified WWII Guns.
Basically, put the Damper.L 44 Nozzle and the Barrel Sight 44. onto the Broomstick/Mauser C96.

"Kill 10 enemies within 10 seconds using the Buzzsaw 42 light machine gun. Unlocks the Old Blood and Guts mask, Light Barrel for the Buzzsaw 42 light machine gun, "Patriot" material and "Captain War" pattern.
"
Yeah, this is the one that gave me some trouble. Took me until I'm writing this to complete it.

So basically, this achievement can be done on any heist with lots of cops. Hoxton Breakout, especially. Like I said, most of these achievements can be done on Hoxton Breakout.

However, when I would do this, I'd never see many cops, anywhere. They'd always end up killing me, Mendoza's on Firestarter? Too dispersed, downed me as if I was made of paper with paper armor on.
But, there is one heist I've seen nobody cover that works perfectly with this achievement.

UNDERCOVER.

It's a Wolf Pack Heist. Single Day, best done at the bottom left of the staircase, even better if that's where you're objective is.

So, cops can swarm the staircase area near there if you're objective is there. They all pile up. Literally into eachother. I've gotten another achievement on this heist as well (similar, from the BBQ Pack, with the Piglet Grenade Launcher).
Unfortunatley, not enough cops piled up there to give the achievement easily, but the cops were around the area.
I did this on Hard, but I think it can be done on any difficulty. Easier if the enemies have lower health. Weak Cops, Blue Swats, MPD Swats, doesn't matter. Could even be shields if you've got the right skills to knock them down.

This is one I'm about to do once I finish writing this guide I finished the achievement with some randoms last night, if they read this guide, I want to thank them. But it's eaiser if you have a group of people who want to do it. But it can be done if you've got a bunch of randoms who have the masks. Doesn't matter if two of you have the same mask, just play the heist. Finish it, get the achievement.