After a fair bit of experimenting, he makes two discoveries; the first is that the 'burst' effect he's looking to create, when coupled with his shield, simply won't work. The shield spell is tied too tightly to the item, that it doesn't have the surge effect he's looking for. What he does discover, though, is that if he pours more energy into it, the shield can hold it, pretty well. He won't be able to do much more than defend, since it takes a lot of energy and concentration, but he ought to be able to shrug off all but the worst attacks.