I like to teleport the lich to an enclosed space (bedroom or some other room) and then beating him senseless with melee pawns. To do this you can use arcane mages or you can swap with faceless. If the lich dies, the undead will too.

The lich can't defend itself in melee range. In other words, melee is your best weapon.

For me the trick is to rush his group immediately with all combat pawns once you get the message. That way he doesn't have time to summon more skeletons. Getting into melee range quickly also helps, because it stops his very lethal death blow spells.

Shoot it with Sniper Rifles. Works every time.
A little micromanagement is all you need if someone accidentally gets in range of the death spells. The projectiles travel slowly, so there's plenty of time to run forward or backward.

I've got a Super Soldier with Rifle Spec and an Anti-Material Rifle. He can reliably two-three shot Lichs.

Anyone aware of any plans to occasionally have a "wandering" lich spawn in the ancient danger instead of mechanoids?

Hit it till it dies!

Jokes aside, if you can lure the large skeletons away, you can easily mow down the small skeletons and lock the lich into Melee combat.

Either use explosives or focus the lich hard. Once he dies, all the undead will too. Dropping all your ults on the lich is also usually a good idea.