1) Never lose echoes.
2) Use the solid echoes only to "round up" so to speak, to push each trip to the dream to the next level.
3) That boss is pretty easy regardless of level, because none of the fight depends on anything related to your stats or weapon, in all honestly.
4) If you explore
early, you're likely to get a bunch of levels our of that.
5) "Half of the side stuff"? What did you do exactly? Most of the bosses in the game are optional, and there's a handful of optional zones. Each boss gives so many echoes that you'll surge ahead in level each time.