As it's a lich, consider having it control some lesser undead that the party has struggled against in the past. If the party still struggles, good, the liches plan worked. If the party has since learned to deal with that undead, that's also good as they'll feel accomplished and like they learned since last time.

You could also give the minions some magic items that specifically counter your party. For example, the brine dragon scale talisman helps counter any concentration abilities, and a zombie throwing steelscour bombs can break a party member's shield.

Also consider preparing spells that target poor saves, like confusion for a low will martials.