Greetings, this has nothing to do with with mnemonics, a field in which I’m not qualified enough to advise you, and I have never learnt Russian, but I would advise you to not use duolingo. There are plenty of quality ressources and websites, sometimes completely free to improve your Russian online that are downright better than Doulingo. I would recommend the use of Pimsleur to better the pronunciation, the use of the app tandem if you wanna chat with Russians and hone your sppeaking skills while meeting very interesting people, you really practice and find new friends there, and it’s free. FluentU is reliable, also.
But the Best is always to go listen to natives directly on the russian speaking side of the internet, if you’re willing to, go to vkontakte, watch Russian Youtubers, read reddit threads maybe… That’s what worked for the languages I learnt.
With duolingo I reckon you’re going to learn useless sentences and get bored quickly.
Have a swell day.