Ryan at LegitMTG has a great article on how he broke into the Mythic ranks. Ryan's story isn't unlike most Mythic ranked players.   The major points are that it takes grinding hundreds of hours to reach Mythic. The rewards for that time spent are negligible with the excpetion of being able to say you made the Mythic rank. If you are after cards to build decks then the grind won't help you as much as just playing to acquire cards through events, daily quests, etc. 

And so after grinding for dozens upon dozens of hours, with a variety of decks (both brews and meta), and going from Bronze Tier 4 to Top 50 in the Mythic rankings, I can firmly tell you what I feel is the optimized road-map for achieving success in MTG Arena in its current best of 1 format.

Players coming to MTGA from most other games will be familiar with the best of 1 (BO1) format, but Magic has historically not been played that way.   Magic used a best of 3 (BO3) format with a sideboard that allowed card changes between games.   BO1 magic is a newcomer, but with a $10M prize pool it has everyone's attention. 

New players will soon read about and play against a deck called Red Deck Wins.   It's a mono-colour deck made with... you guessed it, red cards.   It comes at you fast and furious and in a one-off winner gets the prize format it is formidable.   RDW games are also quickly over, win or lose.    That means you can grind faster.   A 55% win rate where you play 10 minute games is much more effective than a 55% win rate where games take you 20 minutes.  

On top of that red affords me smoke breaks and mental breaks and those are two things I enjoy, also bitching about other people nut-drawing, I can’t imagine not being able to spend 30 minutes bitching about people nut-drawing. So that’s why I was on red. I’m sure someone else was on red ‘cause they found it was the optimized deck for the format. That’s great. I was on red for my health.

 

If you want the badge of honour that is making it to Mythic and let's face who doesn't really then this article is a fantastic read.  

You can live through Ryan's entire journey to Mythic at LegitMTG: A ROADMAP TO MYTHIC: HOW I BECAME ONE OF THE TOP 50 RANKED PLAYERS ON MTG ARENA