Since this achievement is the longest one to get (along with the level 200 one, depending on your typing skill), you should focus on it early on. All the other ones will unlock naturally and don't need special focus if you follow this guide.
You are given the possibility to attack with Mirko as soon as the pink bar, at the botton right corner, is filled up. He will appear just over the middle "normal" attack.
There is a unique way to fill the pink bar up : you must attack ennemies with their weak element. There are 5 elements in the game, each of those are represented by a colored disk near the name of the ennemy and of your characters.
To help you out, check out the elemental scheme in the upper right corner. You can quickly understand that :
- blue ennemies weakness is the green element
- green ennemies weakness is the red element
- red ennemies weakness is the blue element
- yellow ennemies weakness is the purple element
- purple ennemies weakness is the yellow element.
Everytime you hit an ennemi with its weak element, the pink bar will fill up. You need to do it seven times to fill it up completely and thus attack with Mirko.
Only three characters can appear at the same time. Every time you attack, the attacker will disappear from the list, and a random character from the three reserves will take its place (although the same attacker can appear again immediately).
The strategy is very simple : attack every ennemy with its weak element if available. Obviously, when Mirko appears, attack with him.
If you plan on getting all the achievements as fast as possible, don't bother saving Mirko for bosses since you'll get the "Stage 100" way before you level up to 200. Just focus on attacking with Mirko and the correct elements.