Literally the only advantage from casting offensive spells is to out resource your enemy like Eragon managed to do in his second battle with Murtagh. And for that literally the only spell required is "Hold in place". And even if wards didn't exist, no offensive spell save Oromis's remotely cutting an artery in the brain, could ever be more effective than accelerating a pebble to the velocity of a bullet. Brom critisized Eragon for trying to kill the urgals with a flashy lasery spell on their path to Dras Leona, but I question why Brom even though Eragon a spell like that when the bullet pebble spell is more effective, less energy intensive and easier to learn.