Natalie has a sub weapon that absolutely destroys bosses, the raging firestorm.

It has a mechanic where if touches any hitbox, it will destroy itself and deal damage. However, if you hold the x button to summon the firestorm while touching the hitbox, it will keep spawning the firestorm until the enemy with the hitbox dies.

To get the best effect on a boss, run into it and hold x. Then, make sure to keep an eye on your health as it will fluctuate up or down as you gain hearts from getting perfect waves and lose it by taking damage.

An important note about this trick: It only works while you have access to keyboard controls. Also, if you are using just keyboard controls, you CANNOT MOVE while doing the glitch. If you do move while on keyboard, you will have to stop moving, release x and then press it again to reset the glitch. This works fine for most maingame bosses (expansion bosses not so much), but you will end up losing some time due to the boss moving out of range. To counteract the timeloss, start your bossglitch in a location where the boss doesn't run off as easily. For example, if the boss moves left first, start your boss glitch towards the left of the boss's hitbox.