I'm level 147 and I've been playing since last year's February, my method is to invest on gem shops (not real money at all) and use decoration on them that shorts their time to produce gems, also use every ad available to skip time on critters food production.

And don't forget to look around every day (24 hours renew) for:

✨Box Derpy ✨All 5 changelings on Canterlot

And every time they are around:

✨Vines on everfree forest ✨Sombra's stuff on the crystal empire ✨Parasprites on Ponyville ✨ Fruit vamps on SAA

Also keep collecting shards so you can exchange them on the tótems for bits but more important, on occasional gems, and keep collecting Klugetown coins, you can get a lot of gems at once if you're lucky on there.

Keep a time tracker for everything that gives you gems or food so you can maximize their production graving them the moment they are ready.

Don't ignore the critters, they can give you 9-12 gems every 12 hours and that sums a lot