Yeah, generally you'll want to fight monsters that are lightning based (use earth), fire based (use aero), or water based (use lightning from THM). You do NOT want to fight monsters that are earth based (they resist both earth and lightning), or wind based (they resist aero). You CAN use fire against ice-based mobs, but make sure not to get yourself killed with the AoE.

That's not to say you CAN'T kill monsters that resist these, but they're going to lower your DPS options and/or put a bigger drain on your resources. On that note, feel free to pull out your weapon, especially if using a wand and shield. Shield blocks don't stop you from casting, and in either case it's a little bit more DPS. Just a little, but DPS is DPS is DPS. It does stop you from regenerating MP, but I'm pretty sure that doesn't happen if you've got enmity anyway. Just put it away between fights.