Gems of War

You can craft badges/medals by collecting tokens after beating a mythic boss fight in explore mode. There are 6 different tokens and if you collect 3 of the same, they transform to a badge that you can equip in one of the 3 slots you see in the diamond shaped slots above the main interface. All of the badge/medal effects can stack and this will be pretty handy in other events or delves, but for the achievement we only have to equip a single badge anyway.

To be able to fight against a mythic boss you have to collect 100 mythstones. You have to pick a difficulty before you do 4 normal explore fights, the chosen difficulty raises the gold, mythstone and token income. Retreating a battle will break your current progress, losing a normal battle will just give you the option to end your run or to retry. Much worse is to lose against a mythic boss, because you lose your progress + invested Mythstones. Each unlocked difficulty is shared between all kingdoms and mythstones drops from the mini boss fights. The chosen difficulty shows you the range in which amount they can drop.

Look out for a symbol above a kingdom for 20% more mythstone income, but depending on your own team stats it's maybe also a good idea to pick a weak kingdom or a kingdom where you need specific traitstones. As example: I started a level 150 mythic boss fight in Whitehelm and Gaards Avatar wiped nearly all my troops with a "deal 140+ damage to all troops" spell. Some other kingdoms have weaker mythic cards like the Grosh-Nak one.

This time there is no limitation beside of your own time. You can fight as many times you want. If you have trouble with really high difficult settings, pick a lower one. Difficulty 10 (Level 100) already give you the best amount of possible tokens after a fight. I will improve my solution with some more team suggestions very soon and be happy about any other team builds in the comment section of this solution. My first suggestion is the team i used myself most of the time. The other teams are alternatives you can try out if you want.

TEAM SUGGESTION 1
Type: Gold Based
Benefits: Good and fast damage output, many respawning troops, more gold after battles
Disadvantages: If your Hero dies, it's hard to win.
Banner: Meteor Banner
Cedrik Sparklesack
Egg Thief
+10 Skeleton Key (Thief Class Weapon that unlocks with 250 wins)
Class: Thief > Hunt, Knife Throwing, Light Fingers, Backup, Stealthy, Dodge, Rising Shadows)
Greed

TEAM SUGGESTION 2
Type: True Damage based
Benefits: Scaling damage output, Good for Difficulty 12, High Defense
Disadvantages: Preperation with the Shield takes a bit time.
Banner: Kraken Banner
+10 Shield of Urskaya (Sentinel Class Weapons that unlocks with 250 wins)
Class: Titan > Impact, Hammer Mastery, Thunder Fist, Storm Aura, Rock Solid, Stone Mastery, Fortitude
Dwarven Gate
Tesla
Tesla

TEAM SUGGESTION 3
Type: Mana Creation
Benefits: Extra turns, very loopy
Disadvantages: Not good against Level 150 due the small damage output.
Banner: Sorrowful Banner
+9 Elemenetal Bow
Class: Titan > Impact, Hammer Mastery, Thunder Fist, Storm Aura, Rock Solid, Stone Mastery, Fortitude
*other classes probably helps more, feel free to change if you have some more fitting*
Forest Troll
King Gobtruffle
King Gobtruffle

TEAM SUGGESTION 4
Type: Ice Based
Benefits: Freeze kills extra turns, good mana flow if class get extra blue mana
Disadvantages: Useless without Level 70 Frostmage
Banner: Frozen Banner
Daughter of Ice
+10 Orb of Winter
Class: Frostmage > Snap Freeze, Insulated, Anti-Magic, Mana Source, Deluge, Water Mastery, Mystic Channel
Queen Mab
Skadi