Quote Originally Posted by Eriol View Post
Not necessarily IMO. Some of the earlier answers (like Sickening Radiance) also offer another path: burn through the resistance. In many ways, Phantasmal force is almost as good as a recurring-save spell like Radiance, in that it requires a save, that they either burn a legendary on, OR they have bad stuff happen which requires them to use their ACTION to break through (not a legendary resistance, thus you're turn-denying them too), and all for a level 2 slot. That's pretty nice.

So I wouldn't discount the utility of using "decent but not horrific" low-level spells to burn through a big bad's legendary resistances. Just bad enough to get the DM to consider burning the legendaries for can be worth it.
The only thing more fun than burning through LR and landing a crippling spell like Feeblemind, is convincing it (or the DM) that there's something worse than whatever it's currently saving against and finally killing it with its LR (and your high level slot) intact.