In League of Legends I am Gold 5 or 4 (depending on when you will read this). My MMR was really high since I had more than double of wins compared to my losses. This made me play against high gold and low plat players. However, I was on a losing streak and I started playing duo queue with a friend that is Silver 5. This drastically dropped my MMR. If I want to raise my MMR again, does it help to just duo queue with a friend which has higher MMR than me (let's say high plat), or is winning solo queue the only solution here?
Duoing can help if the player you're playing with is dramatically higher in rank, otherwise it wouldn't work for elo boosters. If you're close in MMR, it shouldn't matter too much to your MMR. I think it's more likely that you and your friend had a bit of a bad streak.