Having not obtained this using the other methods, I discovered another route for upgrading your troop to Mythic.
This method is dependent on playing either Dungeons or being in a Guild long enough to have obtained Orbs as guild rewards.
On the Troop Upgrade tab, tab over to "Orbs" tab; some orbs will instantly allow you to upgrade Epic or Legendary troops to Mythic.
In my case, an "Orb of Ascension" allowed me to level up one rarity, and bring my Legendary straight to Mythic, without any need to ascend in the regular fashion!
Side note - I was then able to unlock Level 20 for 1,000 souls, unlocking
Hope this helps someone, let me know any questions!