It's not true ... I have started all over again my run on 3DS and I managed to finish the EB (thanks to Angry Zygarde) and all main stages in less than 3 months
Some people have stuff to do that
actually matters, like a job, school work, or kids. This is just a casual puzzle game, there's no need for inflammatory posts like this.
DHR-107, Mega Glalie and Mawile are huge walls especially because there's not very much available early game that hurts them. When you go back later (perhaps for an S-rank if you don't already have one) with stronger mons, you'll find very quickly that they're a cakewalk because of what's available after them. As for your issues with Glalie now, as I recall grinding+all items is the way to go your first time through. Perhaps you'll get lucky and Mega Diancie will be re-run while you're grinding for it, since its ability destroys barriers.
If you've caught Zygarde already then don't worry
too much about the additional prizes, just focus on doing the other events this week and then getting through the game really. You'll find stronger mons (aim for S-ranks, because you'll really want to start getting some of the important expert guys like Genesect). I can't recommend the best team available but I'm sure you'll figure it out! As for expert mode, unfortunately most of the time it requires items to clear. I usually go through expert stages with almost all or all items because of how insane they are.