Mythic is the new rarity added with the title update. To get a troop to Mythic rarity you have to sacrifice duplicates of the same troop to itself. The rarity order now goes:
White > Green >Blue >Purple >Orange > Light Blue.
Common > Rare > Ultra Rare > Epic > Legendary > Mythic
The first rarity upgrade costs 5 duplicates. The second rarity upgrade costs 10 duplicates. The third is 25 duplicates. The fourth is 50 duplicates. The final upgrade is 100 duplicates.
So to upgrade a troop that is normally a Legendary to Mythic will take 5 duplicates of that Legendary. If you want to upgrade a troop that is normally an Epic it will take 5 duplicate troops to upgrade it to Legendary and then 10 more duplicate troops to upgrade it from Legendary to Mythic. If you wish to upgrade a troop that is normally an Ultra Rare it will take 5 duplicate troops to upgrade it to Epic, then 10 duplicates to upgrade it to Legendary, and finally 25 more duplicates to upgrade it to Mythic.
One thing that you can do if you have a lot of glory saved up is to go into the story and purchase the Bone Dragon package for 500 Glory. This will give you 1 Bone Dragon, 2 Event keys, as well as some gold, souls, and and trait stones. You can then use the 2 event keys that you just got to open a couple of Event Chests. These chests have a chance to grant you another copy of the Bone Dragon. After you have done that you can purchase another Bone Dragon package for another 500 glory. Then use the keys to open a couple of more event chests. Keep repeating this until you have gotten a total of 6 Bone Dragons. Once you have all 6 Bone Dragons, go to the troop tab, select the Bone Dragon, hit the
to enter the crafiting menu. One in the Crafting menu hit the
and go to the Ascenion Tab. With the
change the number of Sacrifices to 5 and select Sacrifice and your Bone Dragon will change from Legendary to Mythic and your achievement will pop.
One thing to note, the event will change in a few days and the Bone Dragon will no longer be available for purchase. It will be replaced with another Troop that may or may not be a Legendary. If it is another Legendary then you can do the same thing just using whatever the new troop is that has been released.
The other way to do this is just keep purchasing chests with either gold, glory, or gems. Keep building up the duplicate troops and sacrifice them as you get them. Eventually one of your troops will make it up to Mythic and the achievement will pop.
I hope this guide helped. If you have any questions, or any suggestions to improve this suggestion please do not hesitate to leave me a comment or to send me a PM and I will be happy to help.
brought up another good point. For those who don't mind spending real money there is a deal in the shop, under bundles, that will give you 16 copies of an epic troop. Which is how many you would need to level that troop to Mythic rarity. It has the added bonus of giving you enough trait stones to unlock all 3 of its traits. As well as giving you a bunch of points to help raise your VIP level. This is by no means necessary as you can get a troop Mythic without spending any real money, but it is an option, so I thought I would add it for anyone who might be interested.