High level encounter and Antagonist help

[Degwerks]

For a party of 5 players, not sure what class they'll choose (1 is a Hunter Ranger and 1 will be a Bear Totem Barbarian), what level should they be to be able to:
Have a boss fight encounter with a Lich who's a 20th level Lore Bard 3rd level Warlock Fiend tomepact in his Lair?
Have several encounters outside of the Lair, where the Lich doesn't stay and fight for more than 2 rounds? Due to the Lich not being an idiot and doesn't want to die and lose his stuff.

This will be for a small "closure" style of campaign, that kinda ended several years ago.

[Teaguethebean]

Is it actually a lich with class levels tacked on or is it just a Lore bard 20 Feindlock 3 with some lich abilities tacked on

[darknite]

I've had some recent success with liches in lairs that have supporting Glyphs of Warding surrounding a 'Power Circle' containing spells like Globe of Invulnerability, Force Cage, Slow, Hold Monster, etc. Glyphs are extra cool because concentration spells cast through them just last for the entire duration without the need for a creature to concentrate on them.

One of the nastiest things to do is have your lich cast Antimagic Field and go full melee with their Paralyzing Touch. Since the Antimagic Field keeps any spells and magical weapons at bay they are nigh invulnerable to attack - other than someone tossing a condition on them that breaks concentration somehow.

Lastly, what lich would not have a contingent of undead/construct guards on hand to keep enemies occupied while the lich zaps them with magic?

Having Shapechange on deck for a lich is cool, too, for them to pop into when they hit 50% hp. You can use this emulate some nasty monsters to wear down the PCs while extending the lich's hp pool. If possible choose forms with Legendary Saves to make sure they make any concentration checks and switch between forms with this ability as you lose Legendary Saves (though hp will continue to drop as they take damage regardless of form).

[Degwerks]

Is it actually a lich with class levels tacked on or is it just a Lore bard 20 Feindlock 3 with some lich abilities tacked on

It's actually a Lore Bard 20 Fiend-Lock 3 with the Lich abilities. Minus the Lich being the 18th Wizard style caster... but instead having the 23 character levels, paralyzing touch, legendary stuff and Lair abilities etc...

Started off as a joke a few years back, that one of our previous high level characters had usurped the thrown in our kingdom builder campaign. We had talked about going back and finishing stuff, and the DM decided that this character had become a Lich as well. So DM is swapping out to being a player now, and I'm trying to rustle up ideas on how to make the Antagonist a very real threat for the party.