Gems of War

You get a medal after collecting 3 badges from 9 tokens (3 for each badge) of the same type and you can earn tokens from the boss chest after a mythic boss fight. All 3 badges get combined to a medal automically, even if one of the badges is in one of the slots, which suprised me a bit. Now you can use the medal to upgrade any already maxed out troop. This means your troop need Level 20, all traits and the highest ascension level.

Using a medal for a troop gives you a chance to upgrade it and you find this option + chances in the upgrade section under troops. The first medal you award to a troop is bronze and this will unlock your achievement. Attention: The rarity of the chosen troop will affect the success rate. If you just want to unlock the achievement as quickly as possible choose a more common troop and even the regular medals gives you a 100% upgrade chance. IF YOU FAIL THE UPGRADE YOU WON'T UNLOCK THE ACHIEVEMENT! - and you have to collect another medal for another chance.

To use a medal the most effictive way: Use a medal with a troop of the same rareness. I as example got an ultra-rare medal and used it to level up my ultra-rare Sunbird. Keep in mind you also need 10k souls ready to pay the upgrade costs.

If you still have any question feel free to leave a comment and i do my best to help you guys out.

Just in case this is the only solution you check, i include my troop suggestions from the other solution if you still need ideas or a good team. Feel free to leave own suggestion as a comment. If i figure out better setups in the future i will update my solution.

Here we go:
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