For all the potential Juice Galaxy has for a multidimensional game experience, for now, Juice Galaxy is very much a combat-focused game. Seek out the bosses, thwack/shoot/back/suck/etc. them to death, use what you get to defeat the others.
The best I can offer is to look up boss speedruns; or if you want the spoiler-free route, talk to Mr. Fox and ask him for places you can go. Right now he stands out among the graveyard NPC's by telling you- indirectly- where you can find two of the hidden underground biomes which, in turn, will lead you to three of the boss enemies.