If you are having trouble with current battles, go back to the old battles and play them again. You will still earn the same coin rewards as you did the first time you played them. Bank the coins and then use them to level up your monsters, which gradually strengthens them.
Evolving your monsters takes a specific type of berry that differs depending on the monster, both in type and in quantity. To figure out where to get the berry you need for evolving your monster, go to its profile, go to the “evolve” tab and tap on the picture of the berry. A pop up will appear showing all of the necessary info on where to find them.
If you need to heal a monster that has fallen in battle, the more obvious way is to use one of your Revives. However, another way to heal your monster is to watch a video for 20% healing. The videos are free advertisement videos, and are available for as long as you have an internet connection.
To do more than just the main quests, go to the Islands menu. Tap on any one of the events to participate and battle. They tend to be a little bit tougher than the normal battles, but they provide more rewards, and generally very specific rewards too. Complete them all the way through for good crafting items, equipment and other goodies.
Or, go to the PvP mode to battle against other players for rank, coins and other prizes. A good way to know if someone is easily beatable is to look at the experience levels of their monsters beforehand. If they are of a lower level, they will be easy to beat. If they are a higher level, you won’t have an easy time with them. If they are around the same level, then strategy (matching element vs element) is what will decide the battle.
Want to get more berries, but you have more candy than berries and not enough slots for more Mino Monsters? You can sell your Minos for berries. Level them up a little bit first if you have enough coins to do so, and they will end up being worth more when you sell them.