This fight is quite difficult and will almost certainly take multiple attempts to beat, however you have multiple attacks to use against the boss.
Credit to The Living Tombstone for making the original "Spooky Scary Skeletons" song
Credit to OldCandyPlaz for uploading the instrumental (original uploader unknown)
If you are having trouble learning the attacks, or simply want to know what you'll have to deal with in this game, here is all 5 of the attacks and how best to dodge them:
The first attack happens right at the end of the intro to the music, bones come out of the ground covering the whole floor. To dodge it, simply stand on top of one of the platforms, however the lowest one is just below the range of the bones, so beware.
The next attack is lasers being shot at the player from the boss' cyan eye, it becomes a rapid-fire barrage in the last few seconds, just keep moving.
Attack 3 is when bones drop from the sky, these are destroyed when they hit platforms, so take cover.
Attack 4 is when 3 homing bullets come out of the boss' orange eye and follow the player, try to get them to bunch up.
The final attack, Attack 5, is when the boss stops moving and drops down, try to get to the side of the screen when this happens.
The attacks will repeat in the same order until the boss is killed.