Cataloguing the spells like this:
Aura Shield
Power: 70
Volatility: 20
Difficulty: 25
Elements: Chaos
Aura Blast
Power: 100
Volatility: 20
Difficulty: 25
Elements: Chaos
Doing the math, Aura Shield can be used about 153 times without breaking. More specifically, it would use around 0.6% of Matthew's energy. It would be able to take around 30 GJ, which according to Windows Calculator would allow him to block damage from 29,000 slices of cake. More contextually, that's around 7 tons of TNT.
Likewise, each cast of Aura Blast, calling it in at 100 Power with the same V/D values, would reduce the amount of damage that Aura Shield by 5.6%, allowing for about 18 casts before the shield begins to collapse, assuming that it takes no damage. It can also output a decent amount of damage: almost 15 tons. Very few arcanes will go down from a single hit, but plenty will go down from three or four.
I doubt that most fights will even last long enough for anyone to cast anything 18 times, so that's reasonable enough, especially if he uses CQC in favour of magic.