I am planning a level 10 dungeon crawl where I would like the final encounter to be an [archmage.](https://roll20.net/compendium/dnd5e/Archmage?fromList=Archmage&Name=&Speed=#content) At the moment my party consists of a bard, a ranger, and a fighter with the knight subclass from UA (all level 10). I want this encounter to be difficult, if not extremely so, yet not impossible. According to XP calculators and such, this encounter is deadly however it is still within the "max xp" for the party.

Because I am not sure exactly how this will go and don't want either the party or the mage to be killed in the first couple rounds, I am having this encounter in a place where things could be added or subtracted from the fight rather easily. It is happening in a large throne room-type space and I figure that I should be able to feel out the battle after the first round and add other enemies if I need to. I also could simply choose not to use the wizard's high level spells in order to nerf the encounter on the fly.

Is the CR 12 archmage still too powerful for this party?