One of the biggest early-game threats due to its splitting and high accuracy in attacks.
The first task is approaching Jellies in a way that minimizes the dangers of their splitting. This means avoid melee in corridors and in open rooms. Open rooms are an obvious threat because they can just move around to hit you. Corridors are a threat because a Jelly can split behind you, which not only means you have two to face but your exit is now blocked.
The best approach, independent of your equipment, for meleeing Jellies is by standing in a doorway to a room. You want to set yourself up something like the following.
In the depicted scenario, I can melee the Jelly on the left and most of its spawn will be generated in the room on the left. Jellies cannot attack around corners, so all of the splitting will not help it, and it eventually becomes akin to fighting a bunch of monsters one-on-one. Note that it is possible, albeit rare, for a Jelly in this scenario to spawn behind you, so keep an eye out. However, the general case is, they'll spawn on the open room and be unable to attack you.
The ideal case is one that has at least 5 open surrounding spaces for the Jelly. Jellies can be manageable at 4 or fewer (such as in a corner), but this increases the likelihood that a Jelly could spawn behind you. As they are normal speed, if you know it is safe to do so, try to see if you can lead one into the ideal 5-open space over a smaller area.
There are also a few equipment and situation-specific strategies.
- Use axes and spears, even if you may not have enough strength. While they may split a lot more, the amount you can clear is faster, especially with an axe. Axes and spears are even more useful if you can engage them in a door as above, since the extra multiplying won't harm you and you'll hit defenseless jellies (axes hit around corners, by the way). They are easy enough to hit that the accuracy penalty for wielding a normal axe with only 12 Strength is negligible.
- A wand of negation or a scroll of negation both prevent Jellies from splitting. It won't make it hurt less, but at least there will be fewer of it to dole out the hurt.
- Jellies will carry any afflictions, in the same strength, when dividing. This means that if a Jelly is poisoned or burning, any divisions will have half the health but the same amount of burning and poison. As such, it is potent to poison and even burn a Jelly before you start meleeing, as the splitting will actually work in your favor by bringing them all closer to death.
- Jellies do not divide from poisoning, poison gas (as from a potion of poisonous gas or trap), or from burning in a fire (a staff of fire will cause a split from the initial hit, though). You can use these methods to eliminate a Jelly without it splitting. Tricking them into a trap at a distance is often a very potent method.
- If you do find yourself in a corridor, a staff of lightning is more potent, especially at a range. It will cause rapid division, but the damage across each Jelly will be significant.
- They are not fireproof, and explosive swamp gas is always the solution to enemies that are not fireproof. From a safe range.