In order to get super rare monsters and upgrade them, you must find them in all of the different towers especially for higher towers that you fight in the game and while the normal monsters usually give you a monster coin immediately.
Try to fight the hard monsters in any way until you get the monster coin.
After doing so all you have to do is just let your monsters recover after each round in order to save your potions as much as possible and to make it a much less expensive affair.
In order to upgrade and evolve monsters appearance just follow these descriptions
Tap on the down-arrow below a monster in the monster screen so the statistics menu for that monsters will pop up
That stat pop menu will tell you how many of each gem that you need to upgrade your monster.
Remember your monsters have three levels that are normal, elder and master The level normal is being their original form, and master one is being their most powerful form so upgrade all of your active monsters to be the master one.
In order to mix monsters you have to get a monster recipe first then follow these actions
Just complete certain quests and certain towers to get a bonus then enter monster recipes.
After having the recipe, you need to get both of the required monsters filling spaces in your present monster list plus one empty space.
Now go to the empty space and hit the mix button then scroll to the recipe that you prefer.
Try to get more rare monsters and mix recipes by using the Gachamon function, or if you want to do in fast mixing try to simply use the Gacha function for short.
While adding more spaces to your monster list just try to spend all of those tokens that you have gotten by completing quests and subquests during the game.
If you run out potions and gems and you want to get those for free all you have to do is just read
how to get gems and potions cheat in montower legend of the summoners game on android