You will need to level up your characters and upgrade weapons in order to defeat harder characters as well as conquer boss fights.
You have to put the hours into Pocket RPG. There's not a lot of skill involved. Keep moving. You don't have to kill all enemies: just the end boss.
On the other hand the point of fighting foot soldiers and minions is to get you enough experience points to level up as well as earn coins to upgrade your arsenal.
Treasure boxes give you the items you'll need to really hit those enemies, so make sure you get them all.
When you're fighting boss characters, keep moving, else their powerful attacks will likely kill you quickly.
To gain new skills, you’ll need to tap on the skills button (denoted by a star) on the main map screen. Here you can see and purchase new skills for your character. Certain skills will require that you finish other quests first.
Don’t bother keeping around much less powerful weapons if you are just going to replace them.