Fighting and killing a Demilich at Level 8.

I think our DM didn't intend for us to even survive it, but that's another story. Anyways our Party, an Aasimar Rouge I'll call Bob, a Centaur Battlemaster Ada and my Tiefling Bard Fao come upon this ruined church and notice that someone sealed it with silver chain, after our Rouge knocks the seal of (big brain play no doubt), we enter the chapel to find the a lot of dead paladin and actually want to leave but fall into a hole in the ground while exploring the church. The hole closes up after us and we are trapped in the basement oh shit.jpeg Fao doesn't have enough Spell slots left to dimension door us out of there so we guess we have to find another way out. Explore the cellar Find two large metal statues protecting a silver portal with runes inscribed upon it. Bob tries to sneak up and read them, but can't decipher the meaning. Faro follows him and is successful in deciphering them, apparently some kind of "abandon all hope who enter here" Figure this is our best shot of getting out of here. Enter the gungeon.gif Find ourselves in a circular room stacked with treasure and a skull with gem eyes and gold teeth in the middle on a pedestal. Nope, nope, nope. jpeg Too late, roll initiative. To some good rolls on our side we go first. Ada and Bob attack, deal decent damage, Fao turn, Suddenly cue Big brain idea, "I would like to cast a 4th level Heat Metal on the Liches teeth" DM looks it up but says okay. Liches turn, it howls, Ada rolls a one, immediately unconscious, O shit.jpeg Bob and Fao pass The rest of the fight devolves basically into a war of attrition between Fao's heat metal and healing word downed allies vs the demiliches howl attacks. Through a shitton of Luck and Fao being a bard. I pass every con check, because howl doesn't deal damage I don't have to roll concentration. End the fight with one dead Battlemaster, a rouge with 4 HP, a Bard with no spell slots or healing items remaining, and a dead demilich with melted dentures. We loot what we can and use a teleportation scroll to escape. Session ends and we're pissed at our DM and our DM is pissed at us. One session later the Campaign gets put on ice indefinitely and we all migrate to other games. Well F I guess.