DeceasedCrab plays Kero Blaster - Part 14 of 28

And now it's time for a boss rush...

When you go all the way down the stairwell that leads to the rooftop instead of all the way up, you'll enter the basement. There are a ton of bats here, but the Level 2 Kuro Blaster should protect you. Enter the elevator and go down to B2. You'll enter the teleporter room. If you need items, you can get them here. Also, climb up the ladder that leads straight to the ceiling, leap into the ceiling gap to ther right and go to the left from here to find a hidden 1-up.

From this point is the teleporter itself. Talk to the shopkeeper and you'll start a time attack mode. This time attack mode is a boss gauntlet, where you'll fight all the bosses from Stage 1 all the way to the final boss. The same strategies apply, and this time you'll most likely have max hearts and all weapons (except the Kuro Blaster, ideally) fully upgraded. Once you beat the last boss, you win. You'll then get the credits.