Lightning sorceress

Hi,
I am starting a sorceres for the first time. Any advice on how a lightning sorceress should manage lightning immunes?? Should I put some points in an other skill (cold or fire) or get something with a specific item?

Also, I wanna put the point as possible ins strengh, but it not easy to get enough to wear gears. Any advise on on many point I should put in strengh and easy wait to raise my strengh with items?

Thanks!