I tested this thoroughly, the c stick on tilt stick for sure does not allow me to throw it forward while retreating sadly.
Try this. (I don't know if it works because I do not have Wii U, but in theory it should.)
NOTE: *
C-stick is set to Attack.
1.
Press Special button in air and
HOLD any
Attack Button immediately (
/
/
on Attack is recommended)
ATTENTION: Mega Man prepares to throw, do not lose sight of the animation.)
2.
Hold stick to back. (To retreat)
3.
Tap C-stick to front JUST as Mega Man throw the Metal Blade.
4. If you succeed, quickly release C-stick
and
RELEASE Attack Button.
I hope it works, sorry if my English is bad U_U