Gears of War: Ultimate Edition 10

The best and quickest way to get this achievement is to get a decent team of 4 players. Then you want to play Competitive Team Deathmatch. You can play Social too. There are less try hards in social but you shouldn't have any trouble with a party of 4.

UPDATE: Competitive now includes all maps. I can confirm this as I have just played Subway, Escalation, and Bulletmarsh in a row.

It cycles through every map in order and there's no way you can miss one unless it gets skipped. Takes 5 votes to skip and you have 15 seconds to do it.

I just did this for about 10 matches and it never repeated a map. All maps will eventually show up including the rare ones Courtyard, Bulletmarsh, Sanctuary, Process, and Escalation.

There are 20 maps and 2v2 gnasher Execution counts as one. As long as you stay in the lobby you will cycle through every map.