The achievement "Mythic Champion" is basically impossible to get if you are a F2P player. You can get Mythic in Ranked relatively easily with enough grinding. Getting Mythic in Limited is basically impossible without spending an absurd amount of real money into the game due to the RNG of limited games. You have a couple streaks of bad drafts where you don't get good pulls and there goes your chances of getting Mythic. It's not like Ranked where, when the RNG hits you the wrong way, you can just keep playing to dig yourself out if the hole. I think that achievement needs to be reworked somehow with F2P players in mind.