Overeating
That Magus—whose memory Adam had devoured—had the most exquisite and accurate control over mana out of all the Magi who had attacked Magus Julian Stockholm on that fateful day.
The main reason for the Magus' proficiency in controlling mana could be attributed to this painting technique. The core principle of this technique was to use mana as a paintbrush and slowly develop a better grip at controlling the energy.
Adam felt that although this technique didn't directly strengthen his battling prowess, in the long run, however, it did wonders.
After all, one had to know that when someone casts a spell, there would always be a certain amount of mana that would be wasted. Practicing this painting technique gave the Magus such great control over mana that barely any of it was wasted during spell-casting.