SLASH'EM introduces two new properties derived from D&D, the Need +x weapon to hit and Hits monsters as a +x weapon, where x is an integer from 1 to 4. Essentially, monsters with the first property cannot be damaged with weapons with less than the required enchantment. It is effectively enchantment resistance. The second property is mainly of relevance to those who keep pets, in particular, pacifists, as an otherwise strong pet may be unable to damage a particular monster. Note that these two properties are given to both new monsters and monsters from vanilla. It is possible to gain the Hits monsters as a +x weapon property through polymorph, but not the Need +x weapon to hit property.
What exactly the game checks for is dependent on what you are attacking with. If you can hit the monster, your weapon does normal damage. If you cannot, it does no damage, but will still count towards advancing your skill.
The game checks the enchantment  of the wielded weapon. For artifact weapons, it adds two to the enchantment check, and Magicbane always counts as a +4 weapon. If the thus derived enchantment is greater than or equal to the monster's resistance, it can hit. For example, a +0 artifact weapon will hit a statue gargoyle (whose enchantment resistance is +1), but not Vecna (whose enchantment resistance is +4), since it is a +0 artifact, having for the purposes of this check +2 enchantment. As always, there are exceptions:
- A silver weapon will do normal and silver damage against a silver-hating monster
- A poisoned weapon will always be able to hit (although it may become unpoisoned as usual)
- Monsters attacking with artifact weapons get no artifact bonus; only the enchantment itself is used
For ranged weapons, only the enchantment of the projectile matters, not the enchantment of the launcher. Otherwise, it works as for wielded weapons.
If you are wielding an item that can be enchanted, even if it is armor or a tool, the game still checks the enchantment of that item. Thus, if you have a highly enchanted piece of armor, it can be used as a backup weapon against monsters with enchantment resistance.
If you are kicking a monster, the relevant enchantment is the enchantment of your boots. Note that kicking boots do not help towards this, as one might expect, and that Whisperfeet, the artifact speed boots, do not provide a +2 bonus for this check.
Bare-handed attacks count as a +0 weapon irrespective of your skill, with two exceptions:
- You are polymorphed into a monster with the hits creatures as a +x weapon property, in which case you can hit if x is high enough.
- You are a monk: Monks fighting bare-handed hit creatures as a +XL/4 weapon, meaning that a level 4 monk hits as +1, level 8 as +2, level 12 as +3, and level 16 as +4. Note that this does not apply for wielded weapons, i.e. a level 4 monk wielding a +0 weapon will not be able to hit a statue gargoyle.
Magic attacks, from spells, scrolls, and wands, will function as normal.
Finding yourself unable to attack a monster can be a deadly situation, as most of the monsters with enchantment resistance are rather dangerous. If you find that your main weapon is insufficiently enchanted, check the other cases above to see if you might have something able to attack the monster. If neither you nor your pet is able to damage it, then retreat and/or Elbereth are some of the only options. Whatever you do, note that attempting to attack the monster still uses turns. Also note that many monsters from vanilla nethack (notably vampires and liches) now have enchantment resistance.
If you are a monk, in general you will find that the effective enchantment of your attacks rises quickly enough to keep up with the resistance of monsters being generated. However, monks should avoid descending too deep or doing the Chaotic Quest before XL 16, since they will not be able to damage Vecna or the Archons that can appear near the castle.
If you are playing a pacifist or merely using a strong pet to attack monsters, the enchantment resistance can pose a problem. Most easily obtainable early pets lack the ability to hit monsters with enchantment resistance. Probably the easiest route is to get a pet that can wield weapons, and give it an enchanted weapon. Note that an artifact weapon confers no bonus to a monster merely for being an artifact weapon - it must be enchanted. Magic traps in the Gnomish Mines can often leave you with some pet dwarves or gnomes as pets, which can wield weapons. Also, some baby dragons, obtainable from eggs, hit creatures as a +1 weapon, and are reasonably obtainable in the early game. Those who are unable to tame a baby dragon or an intelligent monster would be well advised to save bananas: in SLASH'EM, they can be used to tame many different Y, including yetis, sasquatches, and zruty, all of which hit as a +1 weapon. In the later game, a bullwhip is quite useful for disarming your pet to enchant its weapon, a useful feature in vanilla that becomes almost crucial in SLASH'EM, as there are no tamable monsters that can naturally hit even an Archon, let alone a Solar.