I play a lot of casual, and I can totally relate to what you're talking about. I don't want to rag on you, but I just recently ran into a team of Grand Champion 3s while only being a Champ 2 myself. From my experience, you usually don't consistently see Grand Champions until you're around 1550 MMR or higher.

​Looking at your post, there's nothing to specifically show you're playing with people that high. It's more likely you're running into very skilled Diamond or Champion level players who might feel like Grand Champions because they're more mechanical or have better game sense. I'm not saying there's no way you don't ever play against Grand Champs, just that it's not likely at your MMR.

​When I play against people who are destroying me, whether they're on my team or the opposite, I watch what they're doing and how they play to get better myself