Assuming you're knocking them out first, their best bet is Magic Missile targeted at themselves. It won't miss, and it's enough hits to kill them instantly. The players could use Dispel Magic to get rid of Contingency, but Glyph of Warding protected by Nystul's Magic Aura would be a bigger problem. Luckily, Nystul's Magic Aura only works on objects, not surfaces, so make sure to move everything not nailed down. Alternately, use an antimagic field, and then carry them out of their lair before dropping it.

The lich could also have a task force to find and destroy their petrified body. Ideally you toss it into a Demiplane, or better yet, True Polymorph them into something with a really low Wisdom, then Imprison them in a Hedged Prison and destroy the special material component. But that takes high level spells and you'll need to rest to get them back or bring the statue to someone that can cast them.

But if a lich is smart enough to set all this up, it's still easier to try to get around it than go after their phylactery. Though you may as well try casting Demiplane and look for a plane where the contents are the lich's phylactery, and the purpose is to hide the lich's phylactery. It's probably also going to contain a ton of traps, which will either go off or prevent you from going there in the first place because you didn't guess all the traps.

Jarnathan?

So... I had the idea that the villain of my game would be a sort of "folk hero" who wrote (and often greatly exaggerated) records of his adventures. Being a bard, he found a way to achieve eternal life through his stories; as long as people kept reading about his adventures, he would maintain his undeath. Worked great until he realized he would need to create new stories to keep people interested. No problem, he could just create a demiplane where adventurers would come in search of treasure, and transport them into perilous and outlandish situations. Now he has new material for his stories! Hopefully they don't catch on to the fact that these stories are made up... because then he'd have to make sure they could never leave...

Pre-Voldemort Voldemort.