Ah, I remember when I had this problem. You may want to sit down, you're about to hear something that may shock you as it once shocked me.

Blasting sucks.

No, really. Dealing direct damage may be the least-useful thing you can do as a spellcaster, even in the absence of such resistances and immunities. If you want to fight demons and devils, you do it with control spells like Grease or Wall of Iron, by buffing your allies, by summoning creatures to fight for you. And all of these spells are better for any combat than blasting will ever be. I suggest investing in some non-blasting options.

You can also ask your DM about Searing Spell (D&D 3.5, Sandstorm) if you REALLY want to blast.