Middle-earth: Shadow of Mordor

A great place to try for this is during the main story mission Press the Advantage. It's found at the bottom right of the map in Southern Udun Pass and is available after you beat the campaign's starting mission. In Press the Advantage you'll be surrounded by uruks and will need to brand 15 of them. Afterwords you'll need to last for two minutes while being attacked by a constant stream of uruk reinforcements while always having at least 10 branded ururks.

You're going to be using Wraith Flash to mass dominate ( + when the combo meter is red), so you'll want to do your best to build a high combo, as a higher combo results in a larger radius and likely-hood that an uruk will be branded. Unlike Talion or in The Test of the Ring Celembrior can only use Wraith Flash (or any red combo moves) once, so you'll need to hit uruks a few times to get your combo meter red again so you can use another Wraith Flash. If an uruk is about to attack you you're better off counter/evading, as it's possible to get hit in the moments before Celembrior slams his fist for the move. Remember, your first Wraith Flash at a combo at X5 isn't going to do much, but you're going to be seeing very nice result at X30 and up.

For the mission itself you'll want to hop down and gather some uruks around you. Once you've got a following build up your combo and use Wraith Flash. You'll now need to build up your combo to red again. Fortunately, you'll get some progress to this with the uruks you stunned from the prior burn. Do a few sword slashes (or counters/evades if you're attacked) to get back to red and use another Wraith Flash. If possible you'll want to angle your attacks towards a branded uruk, as attacking a non-branded one may kill him and leave you with one less chance of a possible brand. You'll be repeating this until you run out of uruks that can be branded, so pay attention to the mini map. You'll have more uruks flooding in to fight you from the north and south, so just roll over to them and continue what you're doing.

Edit: CanadaAckis has made mention of using knives to help build your combo back up. Using these can provide a bit more safety as they're fast to use, so keep them in mind. R A W Collings has mentioned that you can use The One Ring to spam Wraith Flash for it's duration. It's best to amass a large group of uruk before doing so, as this will greatly increase the ease of branding 20.

Remember that the mission will be completed once two minutes have elapsed after you've branded enough uruks to have an army of 15. If you're near the time limit just quit out of the mission and start it over from the beginning.

Edit: As mereel1 mentioned, you don't need to have 20 branded uruks at once. You just need to brand 20 in 60 seconds. Thus you don't need to worry if some die, be it by your hand or an enemy's.

Edit: ThePaleHorseman has listed another excellent way to get this achievement in the comments. If you find success with that method make sure to send some thanks in his direction!