Need to destroy a lich's phylactery

[Grogg of the North]

If the phylactery is an artifact, then it's probably a major artifact. That means there is only one way to destroy it. So you'll probably have to dig through the lich's notes and cast a lot of divination to determine that way. To deal with the lich as it regenerates, go bury the phylactery somewhere safe. Lich regenerates, gets crushed, dies, repeat.

Another option, if they exist in your DM's world, is to find a spellgaunt (MM2). They have the ability to eat magic items, including artifacts. There is the slight downside that when a spellgaunt eats an artifact it may kill them.