You first need to evolve and adopt 10 different Legendary monsters. You can only have two monsters adopted at a time (unless you buy more slots with jewels/coins), so you will need to transiently earn this achievement. This means you will need to come across a Legendary monster, evolve it in the game, adopt it, then use it in a single game.
Each Legendary monster costs 2 jewels to use in the Forest world and 4 in the Desert world. To minimize the number of jewels you use, you should use all 8 of the Legendary Monsters found in the Forest world and 2 of the Legendary monsters found in the Desert world, for a total of 24 jewels. It is best to write down which Legendary monsters you have adopted and used to make sure you do not use the same one twice, which would be wasting jewels. Actually getting the specific Legendary monsters you need will take a bit of luck, but just keep at it. Playing the latter levels of the worlds will give the best odds of being able to findLegendary monsters (Level 15 has a 90% chance of a Legendary monster spawning).
Thanks to cccuaresma1 on TA for this list of all of the Legendary monsters in the Forest and Desert worlds! Check them off as you evolve, adopt, and use each of them.
FOREST (Cost 2 Jewels to Use): Flower, Billie Goat, Monkeyboy, Tessie, Bonbon, Big Bear, Biscuit, and Tiger Toes.
DESERT (Cost 4 Jewels to Use): Ginger, Olive, Hunnybunny, Duke, Lightning, Lana Banana, Indiana, and Gumdrop.