Like other demon lords, Juiblex can and will teleport to meet you. Dealing with him is quite simple, you only need a unicorn horn, a wand of digging, and a weapon of some kind. When he engulfs you, zap him with the wand, then hit him once and he will die. This is because zapping a wand of digging from inside a monster causes severe damage to that monster; their HP drops to one. The engulf attack will almost certainly make you ill, which causes a delayed instadeath, so don't forget to apply the horn (or use another method of curing disease).
Juiblex is one of the demon lords who can be summoned and made peaceful, when a Chaotic player sacrifices his own race on an aligned altar. This means he can appear quite early in the dungeon. If this happens to occur, it may be worth attacking him as soon as you are prepared, because he offers a relatively large XP reward for minimal risk.
If you allow Juiblex to engulf you, and then damage him a bit, he'll teleport (with you inside of him) to the up staircase. This allows easy traversal of his no-teleport swamp lair, especially when coming up from the Sanctum with the Amulet of Yendor. It may be possible to continue this process indefinitely.
Juiblex presents a difficult problem for extinctionists. He cannot be stoned, so repeated stoning and unstoning (which works with most other unique monsters) will not work. He will not pick up an amulet of life saving. He will never leave a corpse, so undead turning also fails. Juiblex is the only denizen of the dungeon that can be killed precisely once, except if by random chance he is generated with an amulet of life saving.
Little is known about the Faceless Lord, even the correct
spelling of his name. He does not have a physical form as
we know it, and those who have peered into his realm claim
he is a slime-like creature who swallows other creatures
alive, spits acidic secretions, and causes disease in his
victims which can be almost instantly fatal.