Right. Forgot readying a spell takes concentration.
So Dimension Door 70' away.
They DO get 1 legendary action, but I don't know if any of them would be useful at that range. Vecna's reaction doesn't trigger.
I walk up and kill them.
How good are you at scouting?
Not good enough to avoid being seen by Vecna who has Truesight to 120 feet and actions that can reach out to that range as well.
You told me I shouldn't have any allies. Or do allies only count if they get in the way?
I did not say you should not have any allies, I said relying on your allies to make your spell work is highly situational, especially when you are expecting them to give up their own actions or higher level spells.
One guy who cast buffs and dimension door doesn't need to move any closer.
But at the level you are at who is that and what is the opportunity costs and you do need to get close for the best buffs in the game.
Take a Cleric for example, the best general Cleric spell buff in the game is Holy Aura and the best situational one is Antimagic Field. The Cleric does need to get close to do either of these and there is an opportunity cost to not doing that.
You are asking a Druid not to cast Shapechange so you can make CME work.
120' EB +30' spell sniper is more than her 120'.
Obviously i won't get CME, but I can kite too.
Yeah but you are doing minimal damage when you do.
But I like how your argument is now "CME isn't broke because no one can get within 15' of legendary creatures."
That is not my entire argument. It isn't broke because you have to be within 15 feet and generally have to hold on to concentration for 2 rounds.
96 damage/ 2 /2 = DC 24 concentration checks.
Vs +9 and advantage = 51% chance to maintain concentration even in the worst case.
Yes, if you have proficiency and Warcaster most of the time you will hang on to concentration for your precast CME but also most of the time you will be Frightened and unable to move closer to Vecna .... and your steed will be dead requiring you to use extra movement to dismount.
Can I assume a Ring of Protection or any other magic item?
Sure, but keep in mind like feats and ASIs, these are limited and you only have 3 attunement.
You are creating a house of cards here with specific feats, specific multiclass combinations, specific allies and the actions they will take, betting the farm on being able to scout Vecna or another legendary creature and precast a host of spells, all to possibly cash in for a ton of damage. These choices are specifically to make something workable at level 18+ but you will have to play the first 18 levels with these compromises.
There are times it will be effective and for that reason it is probably worth getting, but it will not be generally effective at vey high level like you envision and other spells will generally be more effective against those enemies.