There are a couple things you can do offensively. The first, as already noted, is to make the enemy Wet if you see him conjure lightning elements frequently. The easiest ways to do this are either the Rain magick [QQF] or Large Splash Bombs [QQQQD]. This will force the enemy to ether stop using lightning, or more likely alter his resistance or activate a personal shield to counter getting wet - which you can exploit offensively.
The second, more devious plan is to cast the Nullify magick [SE] when he is in the middle of a complex conjuration sequence. This will remove all shields, walls, buffs, and more importantly, already conjured elements - leaving your opponent with a dud spell. While he is trying to figure out how to add the missing elements again or get rid of it and shield himself again, you can move in to attack with basically anything. He will have no armor, wards, or magic resistance aura, and it will be difficult for him to create one quickly with a few random elements in hand.