I would like someone with actual experience against Legendary Creatures with legendary resistance at higher level...between 5th and 15th?
The opponent did have Legendary Actions. The Darkness was a legendary action. It attacked on other character's turns. Martials, especially fighters, can do a lot of damage in a short amount of time. Action Surge burst damage hammered it. We were level 5. 3rd level is the highest spell available. The creature did hit hard. So did the party. The monster is in a module. Not interested in giving away spoilers because it is unnecessary to answer the question.
A group with bless can hammer on a Legendary Creature missing very rarely. The bless spell is very, very powerful. A fighter can wander up use Feinting Attack with bless hitting for 2d6+13 with Great Weapon Mastery. Bless allows him to tee off with Great Weapon Mastery for a lot of damage. A smiting Paladin hammers for a lot of damage quickly and if he uses Vow of Enmity he gets advantage on every attack. Even the darkness reduced him to regular attacks rolls.
Buffing the party was highly effective. Casting spells on the creature not so much until its hit points were lot enough to be affected by sleep.
I'm asking people with experience if this is also what is happening at higher level. Do wizards or casters have to rely on no save spells waiting for the martials to do enough damage to put them in sleep or similar type magic range? Due to Legendary Resistance, I feel my natural tactical options are to hit the creature with low level save or suck spells hoping it fails and is forced to use Legendary Resistance to set up a higher level attack. It appears to force a player into a meta-gaming situation where he must use player knowledge to effectively use his character. It seems like a very strange mechanic forcing a certain type of player rather than character tactical choice that lessens role-playing well. A wizard or other caster must feel absolutely pathetic having his best spells resisted automatically every single time he faces a Legendary Creature. I expected Legendary Creatures to have high saves, and they do, but to auto-save is basically a message to casters that "Your spells will never work unless they don't allow saves against a Legendary Creature." Three saves is huge in a fight of that kind where every class is going all out with their class abilities to win.