Question from a new DM about the CR of the boss.

Hello.

i'm a new DM, running my players through a game i'm writing myself.

my players are a Monk, a Druid, a Warlock and a Pally.

for the boss battle at the end, i will be having an ancient red Dracolich, (CR 23) and many Kobold minions (with 1 hp)

my question is this: i am giving them each a winged mount that can also attack, and will be on par with their level, (such as a Ki-rin)

so what level do my players need to be that the Dracolich wont just cream them no question? what is a good level for them to be for this fight?

thanks!