Narita Boy

Story related, can't be missed.

Once you've acquired the first memory floppy in 'The Room of Three Switches', head one screen to the left and interact with Baba's cave. Inside, she'll ask you to enter the red portal behind her to see what's going on. Lord VHS is waiting for you and it is a very simple battle. Hit him a few times and back off when his sends his shockwave attack towards you that are easy enough to jump over. He will then disappear and reappear so you'll have to just keep yourself moving and dodge just as he appears to avoid being hit.

Watch my video on YouTube

This is the first boss you encounter. The hardest part about this boss is finding him. You have to exit the cathedral and keep heading left to where you came from. Tere by some sheep will be a terminal you can use to access Baba's room and the VHS boss.

Now, for the boss fight itself. VHS has only 2 moves:
teleport - teleports to anywhere in the room, you can see his outline first before he spawns there
One slash - more like a ground attack, you need to jump to avoid it.

The strategy is pretty simple, when he teleports for the first time stand anywhere in the room, preferably in the middle and once you see his outline dash right or left 2x followed by jump to avoid his ground attack. You're free to hit him as much as you want now. Rinse n repeat

For 100% walkthrough or if you get stuck use the video Brain5454 made, all credit goes to him: