[Blissey]

I guess instead of a straight dilution of their HP, it could just be 30%. The vessel's supposed to be vulnerable, and it's 99% impossible for a Lich to die unless they give into Void Poisoning or have their phylactery found (which is impossible). No matter how much they may lose to anyone, they'll always have enough times to return and really hand it to them with their new found knowledge and tactics.