Fire-resistance shouldn't be that big a deal at the endgame since there's a feat tax around it. Fire-immune though, that's the pain.
My calculations also ignored the barbarian having a magical weapon at all. At level 20, what type of weapon is he wielding? A +3 greataxe maybe, with other bonuses? At least I have my +3 -- or wait there's no equivalent weapon for blasters. I wouldn't want to encroach on the poor barbarian's territory!
Also if we're talking endgame, a Dragon Sorcerer needs to cast Scorching Ray as a 5th level spell to beat a high level Agonizing Eldritch Blast in average damage. Then we can add that 24 hour concentration of Hex on EB and the Sorc isn't keeping up, period. And the funny thing is that the Warlock's damage is absolutely fine! I'm not looking to drag Warlock through the mud, just the absolute joke they did here.
Oh, I can do that easily:
"When you cast a spell that targets only one creature and doesn't have a range of self..."
I cast Scorching Ray targeting Bob. Bob is only one creature, and Scorching Ray doesn't have a range of self. Therefore, I qualify for the prerequisites.
Oh, but Scorching Ray could target more than one creature, you say! Therefore if it could then no no no!
Then by that logic, I cannot twin Fire Bolt, because Fire Bolt can target a creature OR object. Because it COULD target an object, this logic states that it doesn't qualify