Solution1:
As DANIELJJ14 Pointed out Starting from Ch4-1 onwards Minions will drop randomly from enemy's
saves some RM.
Solution2:
When your playing the game, grinding levels or playing some happycards.
You often get Star items just like in Happy wars.
These items often have rare metals on them.
What you want is to have 5 rare metals.
Now go to Game Home [RB> and go to Customize > Select Item menu.
And Click : Sell Items. Now sell five parts of equipment with rare metals on them.
Note: the right of the screen when selling items, where u see the amount of Stars u get. just above that says if it has Rare Metals and the amount the Item contains.
When you're finished, confirm and exit sell items menu.
Now u finally got you're rare metals you can exchange them for a Minion.
In Game Home menu [RB> go to shop. And Click Rare Metal Shop at bottom right.
Then [RB> to Minions. Buy a minion of you're liking and Voila!
The achievements is yours.
Hope this helps ;)