Mustard gas, by a means of two five gallon buckets in a large backpack, rigged each with bleach and ammonia respectively, attached to an air pump, mixture chamber, and a motorcycle battery.
I'd send waves and waves of low level minions after you that you could beat, slowly upping the quality of monsters while leading you through a dungeon to only one way of escape. When you realize there is only one possible escape route as things get too tough, you go through a door leading to a portal that brings you to a movie theater which only plays interviews of Hayao Miyazaki and Hideaki Anno kvetching and proselytizing about their lunacies. Enjoy your final moments of life. I'll leave popcorn Snow Caps and Juju Fruits to snack on as you die slowly.