Ok so It's similar to the other solutions here with a small but important addtion. Create a match and make sure that the language priority is is set to Deutch then tell you're boosting buddy, (yes I'm assuming that you're boosting), to search custom matches with the language priority to Deutch and you'll be able to find each other relativly quickly. The round time and the number of rounds really is up to you but it doesn't matter really if he lets you win and vice versa. This also applies to the other ranked win based achievements. And the wins count toward the 100 online wins and the online matches played.
Hope this helps. Please comments if it doesn't and if it doesn't comment or message me so that I can modify the solution.
Cheerio