This isn't really about Mythic. I just want to point to this decklist.
Lewis Laskin UWR Control
The deck looks pretty sick. It runs every planeswalker my Mythic runs + Ajani Vengeant. But the dire problem is 4 Path to Exiles, which can wreck Mythic if you aren't prepared for it. It might wreck you even if you are expecting it. The 3 O-rings are also quite good against my Mythic deck.
I expect Laskin Control to become quite popular quickly considering it just won a SCG 5K. It'll be more popular surely than some random Mythic build I won a PTQ with. If you plan to run Mythic, prepare for this deck.
If you don't play with Mythic, I recommend going with that list.
My general plan against this UWR build after boarding is probably to fight Flashfreeze by taking out Rafiqs and Thornlings, going with more UW spells of your own. Negates seem particularly key in this matchup and unlike against regular UW lists, I would board in all 3 as well as the Dauntless Escorts. (If you don't know what I am talking about, check the previous post. It has sideboard plans in it.)
