So looking at its physical defense for a second (and this where I wanted to check if you were still wanting to stick to WOTC's terrible CR metrics at these levels or wanted an "actual" CR18 monster).
Take a basic champion 11th level fighter wtih a +2 longsword and dueling style (so just a regular old non-super optimized fighter just whacking away). In two rounds (assuming action surge), 3 of these guys would do 267 damage on average.
So with character much lower than the CR, and just 3 of them (which means a CR18 should be x2 deadly against them), I basically can vape this guy in 2 rounds. Not even a full party, and I'm not even assuming any magical spell buffs, advantage on attacks for the innumerable reasons you might have them, etc etc.
This is the most common problem with high CR monsters (that and they don't do enough damage). They just cannot take the pain of even mid-level character.
Take a basic champion 11th level fighter wtih a +2 longsword and dueling style (so just a regular old non-super optimized fighter just whacking away). In two rounds (assuming action surge), 3 of these guys would do 267 damage on average.
So with character much lower than the CR, and just 3 of them (which means a CR18 should be x2 deadly against them), I basically can vape this guy in 2 rounds. Not even a full party, and I'm not even assuming any magical spell buffs, advantage on attacks for the innumerable reasons you might have them, etc etc.
This is the most common problem with high CR monsters (that and they don't do enough damage). They just cannot take the pain of even mid-level character.