Welcome! (:
Try checking the wiki for monster levels. It doesn't seem like the levels are too important, as usually if you're continuously grinding on a monster you'll notice when the monster becomes juuust right for you to fight.
Colour of mobs help a lot actually! xD
I can't remember off the top of my head, but green should be easy to kill, yellow should be suited for soloing/easy partying, orange is anywhere from 3-7 levels above you, and red would be relatively hard to solo (later on in the game, it gets easier to solo red monsters).
Have fun in Fiesta!
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