Basically, as for the Mystic Power levels, looking at Aquaria, she attacked three times, using 3 PP each time, and she only has 17 total. It's the equivalent of a level 4 Sorcerer using all three of their 2nd level spell slots in a row. Same with Terrus, he used two 2nd level spell slots in one turn, then subsequently used another one to make him harder to hit.
The difference is, I suppose, that the Mystic has a little more flexibility in their abilities than a Sorcerer does with their spells. But then, Sorcerers get Metamagic and Wizards can not only recharge a spell slot during a short rest, but they also have a spellbook.