After the first technique of Guardian's attack type technique was explained to Valentina and the other leading figures, Aster's "slash" seemed lacking, well, so long as you don't take into account the fact that he can use his technique multiple times in a single day, but that's not the power of the manual but an effect of Aster's monstrous endurance and recovery capacities.
And that is actually the case, after all the "slash" Aster used had a similar effect to the second level of intent "sharpening" which increased the cutting power of the weapon it was imbued into, meaning that it made it easier for it to cut things, Guardian's technique directly ignored obstructions.
However, with a teasing smile that some girls were really acquainted with and found both amusing and annoying depending on whether it was aimed at them or at some other "victim" of the dragon youth's schemes, Aster simply told them to look out for a little "surprise" he had in store for them later.
