Ph33r my 1337 drawing sk1llz!!!!!111one
Actually, I can draw much better than that, I'm just trying to illustrate a point here.
I drew a big looking stickman to represent your boss, then I used a blue rectangle to represent the boss' collision box, and a red rectangle to show the space being contested, much like in that Street Fighter tutorial I posted.
First, there's the ground stomp (you may want to use a different trajectory than the one my arrow shows, that's not set in stone), with the red box over the boss' feet, extending a bit to the sides because of the shockwave. Then there are the projectiles. For the high projectile, I drew it both straight and angled. It's really your choice which one you want to use, or you may even want to use both.
The combination of a high and low projectile, both horizontal, is actually a pretty common thing to see in the Mega Man X games, with Magma Dragoon
from X4 being a good example. Since the dash lowers your stance, you're supposed to jump over the low flames and dash unter the high ones. Another good example, also from Capcom, is Sagat from Street Fighter. His basic playstyle involves alternating between high and low Tiger Shots, forcing the opponent to duck or jump accordingly. If your game has some way to reduce the player character's height, this can be a good pattern to use.