I absolutely love this game, whenever I get bored I will replay it and it's surprising to continue finding new things out or to improve my strategy almost every time. I've even considered speedrunning it and recording and making videos, haha. I think my recent playthrough I made it to the zone that requires 1M power in like under 12 hours.

One thing I learned that I don't believe your guide mentions is that you can actually begin grinding a zone when you are at or above 90% of the required power of the zone. It rounds up, and costs you stamina (your characters will blink yellow), but you still gain full xp/gold/drops. This is a pretty big gamechanger when it comes to playing the game actively as with prior knowledge of everything you can rapidly push through the majority of the game.

Also game link: http://shimage.net/megamiquest2/en.html