Also spells like Faery Fire are great spells to spam at a creature with LR. It's pretty devastating to them if they don't save, and low enough you can spam it and burn through their LR. Likewise a Battlemaster who multi attacks can burn through a monsters LR in one turn.
I don't know if you mean this from how the post is written, but a Legendary creature can always choose to not use its Legendary Resistance on a failed save. Just failing a save does not automatically cause it to kick in. The Legendary creature can save it for the "bad" spells.