I haven't played a lot of the Megaman games. My understanding is that you can only shoot left or right? Possibly up or down. Regardless, if your goal is to create shooting that doesn't use the mouse it is fairly straight forward.
Instance one: Shooting only in the horizontal direction of movement (left or right).
Code:
///event that creates the bullet
with (instance_create(x, y, obj_bullet)) {
spd = 5;
facing = other.facing; //which I believe Shaun's tutorial tracts, but it is basically if (hsp != 0) facing = sign(hsp);
}
///bullet object step event
x += spd * facing;
That is really all you need. Adding in other directions is a little more complicated, but also not too hard. If I remember right, you should just be able to use the bullet code in Shaun Spalding's tutorial but instead of setting the direction to point towards the mouse, simply set it based upon which keys you're pressing. But the basic idea is the same as the only left or right version. Create a bullet object with a few variables that can move it in the direction you want, then when you create the bullet, set those variables.