Based on what I can gather(and be advised this is pure speculation), the Mages Rainboom is an incredibly powerful attack that can only be used by Elementalists that can use multiple elements. Unlike normal Elementalist techniques that only use 1 element, the Rainboom combines 6(I'm guessing Rainbow's using fire, lightning, ice, water, wind and light). The multiple elements build off of each other to vastly increase the power of the attack. However, due to the extreme difficulty in combining multiple elements(being able to use 2 at once is considered the mark of an expert elementalist, 3 that of a true master) resulted in it being thought to be a myth.



As for how Rainbow did it, I even have a theory/story. After gaining mastery over 6 separate elements, she began striving to create the Rainboom. However, she was unsuccessful until events(the My Little Mages version of the Best Young Fliers Competition) resulted in her flying at high speed while launching attacks of the first 5 elements for the Rainboom. Through luck or design, they were flying in the exact formation required for the Rainboom. Thinking quickly, Rainbow caught up to her attacks and created the necessary magic circle for the Rainboom, using her elemental arrows as moving mana foci, and launching an arrow of the 6th element, thus creating her own variant of the technique, the Sonic Rainboom.



So, you like?