Otherwise, Casters would never waste the effort and resources to acquire high-level Magic suitable for their level, only to learn and refine them from scratch. Wouldn't it be far wiser to just keep advancing the Rings of the Spells they already mastered?
And this rule was no exception for Gauss, though his upper limit was somewhat higher than that of ordinary Casters.
Previously, his usual practice was to advance a Spell by two Rings—raising his 1st-level Spell, Magic Missile, to the 3rd Ring. This was relatively easy for him. In truth, he could even advance it to the 4th Ring, but doing so would be a bit of a stretch, even for Gauss.
It would significantly lengthen the casting time of Magic Missile. Although the Missile's power would increase, since his Missiles were primarily used for wide-area, carpet-bombing coverage, forcing them up to the 4th Ring simply wasn't a cost-effective approach.
But after his Talent advanced, everything changed.
