Sparklite

This is the game's first boss and arguably one of the hardest, mainly because of how under-prepared you will likely be. Prior to attempting it, focus on farming gems to purchase health upgrades and on the run you choose to fight the boss, clear out the rest of the Vinelands first, as you're going to want to take any health, damage or speed related pickups you can find. You may also find an upgrade for increasing your melee damage

The boss itself only has a few attacks: when it stomps the ground, stay on the outside edge of the level to avoid getting hit. It will then lunge at you three times. Run or dash to avoid getting hit, then run up to the stationary part of the boss and get a few hits in with your normal melee attack, before retreating and dodging the next attack. The boss then uses a vacuum to try to pull you in for damage. As far as I know, the only way to avoid this is to have a speed boost pickup but they're fairly uncommon at this point so your best bet is to account for taking damage during each of these attacks.