Buffing the BBEG is the natural response but weakening the PCs is another route. Debuffs can go a long way and can be a problem for the players to focus on and strategize around.

When in doubt, its a [Lich] and there's a [Phylactery] that involves a moral conundrum or sacrificing something or becoming cursed in some fashion: convert the hard rolls a mental one for the players. No natural 20 rolling you way out of willfully giving up your PC's left hand. Or a permanent aura of stench.

Unless they do XYZ of course. Or maybe. They should have listened to that NPC on day one more.