spells that you would never want.

[Elkad]

Yeah, because a single CR = party level enemy is not a tough or interesting fight most of the time. Most DMs run harder fights, which does decrease blasting value, but keeps save-or-die value about the same, because it bypasses everything but saves, and saves don't scale as hard as HP.

Decreases blasting value?

I pad encounters by adding low(er) level enemies quite often. AoE blasting clears those out nicely, leaving just the boss.

Choosing off the CR10 table. Rakshasa. Eh, it'll get mobbed, and DR 15 means nothing vs a mid-level ubercharger. I want to bring the encounter up to CR13ish, and I don't want to use 2, so I go shopping at half it's CR and give it.. 12 gargoyles. Not the best synergy, but it definitely ups the difficulty. They clutter up the battlefield, break up Charge lines, etc. Or fly to stay away from melee (and out of the Stinking Cloud). Their DR works well vs most summons, without nerfing the actual melee toons. The Rakshasa's Haste benefits them. His Silent Image either hides them, or makes more of them. Maybe swap out his Acid Arrow for Levitate, so the whole encounter has an aerial component.

Now you have an invisible levitating Rakshasa buffing his gargoyles and reading the party's minds to adjust his tactics, while choosing his opportunity to intervene directly.
I might even let his Change Shape include Gargoyle as a form, so he can blend in with them. (I have been know to use an ad-hoc unpublished rules exception from time to time...)

A cloudkill or a couple fireballs won't worry the Rakshasa at all. But they will clear most of the gargoyles, or at least keep them from mobbing the wizard, while using up some party resources.