How to Level Up
Take 5 low lvl monsties in your group and fight something of your level. Or stockpile the quests you accumulate over the couse of playing the game and turn some of them in whenever you need to level a new monsties. You can also re-accept subquests!
Send them on expeditions is the fastest if you want to level a batch of them, the other way is just drag them along in your party they will even level up way faster that way. And if you are high enough level go look for the highest level mosnter that you can defeat with just quick battles and your monsties will catch up in no time.