Damage Cut is a English-community colloquial term that refers to any buff that specifically reduces damage by a fixed percentage. It is based on the English translation for Holy Saber's Phalanx skill, which reads "70% DMG cut to all parties."
Damage Cuts are calculated separately from other damage reduction effects, most notably defense. The main significance of damage cuts is that the different types are additive, meaning you can achieve a 100% damage cut, reducing all damage received to zero. This is extremely important in many raids, such as Proto Bahamut and Grand Order, where certain triggered attacks can deal damage in the millions.
Holy Saber's Phalanx provides the baseline, a 70% damage cut against all damage for 1 turn. Many characters also have similar abilities. In raids, there will usually be at least one Holy Saber who uses Phalanx with the correct timing, so you just need to provide another source of damage cut in order to achieve invulnerability.
|Gawain||60 (70)||6 (5)|
Non-Phalanx damage cut effects
Phalanx effects from non-character sources will stack with character sources, so you can use these to reach 100% damage cut. Note that they do count as the same buff type, though; using Athena's call on the same turn you release your Seven-Star Sword charge attack will result in one overwriting the other, netting 30% damage cut, not 60%.
Athena is the most popular option, being a summon stone that can be brought on any team and providing the perfect 30% to complement Holy Saber's Phalanx. In raids, players without Athena can also combo-summon to get the effect.
The Seven-Star Sword, once awoken, has a Charge Attack that also provides a 30% damage cut. However, it's generally difficult to always have your charge bar ready to use when necessary, unless you are using Berserker, Weapon Master, or maybe Seofon, so the Seven-Star Sword is not generally considered a useful application of damage cut.
Skills and summons that grant Elemental Resistance UP are also considered damage cuts, so they are commonly used to heavily dampen incoming damage of a specific element. Because these are completely different buffs from typical damage cut buffs, they stack with the Phalanx damage cuts.
The most commonly-used sources of elemental resistance are the 5 carbuncles, Garnet Carbuncle, Aquamarine Carbuncle, Zircon Carbuncle, Peridot Carbuncle, Opal Carbuncle, and Onyx Carbuncle. By themselves, they provide 25% resistance to their own element, but they can easily be fully uncapped using s from the casino, which boosts their resistance up to 50%. However, be warned that you will only get 25% with a combo-summon in a raid when another player calls a carbuncle.
The "SSR Carbuncles" (Prometheus, Ca Ong, Gilgamesh, Morrigna, Hector, Anubis) provide the same resistance as the SR carbuncles (25%, 50% when fully uncapped), but, since they are significantly harder to uncap, it's less likely that they can be used on their own to reach invulnerability.
Some characters have elemental resistance skills as well.
Last but not least, the Repel buff is, again, a completely different buff, so it stacks with other sources of damage cut. It works the same as normal damage cuts, except that the amount of damage prevented is dealt back to the source of damage instead of being nullified. While many characters have Repel buffs for a single target or support skills, only those below have party-wide Repel buffs.
|De La Fille (Summer)||40||3|
Note that Danua's Scapegoat is not enough by itself to reach invulnerability with Phalanx, but it can be combined with elemental resistance (such as combo-summoning a carbuncle) to get to 100%.