Blade lv. 3 is probably too slow for this fight. Try to downgrade by bumping into a purple critter (not the bullet).
The Blade is not the best choice for Monster X, anyway, because the Bubbler can shoot and protect you from projectiles, but since you don't have it, you can manage with the Blade, anyway.
Firstly, you'll need to kill every enemy in the area, because the Gaudis can oneshot you. Once you got rid of them, you can use that downgraded Blade lv.2 to deal with all the Monster X's bullets (or maybe use the Snake. It might miss a lot, though). The wide hitbox of your Blade should help you in this. Avoid upgrading it, too. (You can do this by bumping into purple critters and healing until your Blade has only half the EXP on lv. 2)
After the green cores have been destroyed, you can simply spam the Blade into it, but do not stay in one place for long. The fish missiles are easy to counter if you run away for long enough.