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A tank is considered uncrushable when they are immune to crushing blows delivered by raid bosses. In order to do this, the attacking mob's attack table must be filled completely with other attack results that take precedence over Crushing Blows. Against an opponent 3 levels higher than the tank — Boss mobs are treated as being 3 levels higher than their targets — all combined avoidance must add up to 102.4%.
Important Notes: As of 3.0, crushing blows mechanism have been changed. Now only mob four level higher than you can deal crushing blows. Therefore, uncrushability is irrelevant for raids in WotLK and the avoidance stats below are no longer necessary.
Calculating combined avoidance
In order to discover a tank's total combined avoidance, one adds together the following values:
- The base chance to be Missed (5%)
- The bonus chance to be Missed bestowed by Defense ( (Defense-350)*0.04%, shown in a tooltip when hovering over Defense on one's character sheet)
- The Dodge chance
- The Parry chance
- The Block chance
Note that Druid tanks cannot block or parry at all; becoming Uncrushable is nearly impossible for a druid. There have been some sets made which push total avoidance (miss+dodge) over 102.4% for a druid, thereby making them uncrushable by pushing all attacks off the table. However, attaining these sets requires a huge amount of effort and access to extremely late end-game gear, and are considered a novelty and by no means a requirement for a serious druid tank.
Even for those tank classes that can parry and block, most still cannot reach this 102.4% value without using some sort of special ability.
For warriors, uncrushability is easy to attain using Shield Block, as the 100% bonus to blocking will make these numbers add up to 102.4% very easily. However, this ability only lasts for 10 seconds, with a 60 second cooldown (which can be reduced to 40 seconds through talents).
For Paladins, it is more difficult, as Holy Shield will add only 30% to this. (Redoubt cannot be relied upon to proc when the paladin needs it.) However, if you know where to look for gear and which pieces of gear to use, it is not difficult to achieve uncrushability with only Holy Shield within two to three weeks of level 70. Most paladins will equip the Libram of Repentance, as this adds a further 42 block rating, or 5.33% block chance, while Holy Shield is active. See Paladins as tanks: Gear Mechanics for other useful tips about paladin tanking gear. See also Karazhan Entry Set, which is a sample uncrushable tanking set that consists only of pre-Kara non-boss drop gear.
A tank is "passively uncrushable" when he or she cannot be crushed, even when not using either Shield Block or Holy Shield. Achieving this level of avoidance requires an enormous amount of avoidance stats, usually by stacking block rating and defense. It is now obtainable through a combination of Karazhan, ZA and Tier 5 gear, although at the expense of significant amount of stamina.
Passive Uncrushability is especially helpful to warriors, as the two charges from Shield Block are eaten up very quickly in many situations. Moreover, passive uncrushability helps the threat of warriors, as they free up rage that otherwise would have been spent on Shield Block. The Warrior talent Shield Specialization makes this easier to attain for warriors, as it gives them an extra 5% block.
It's less useful to paladins, as their eight charges from Holy Shield are used up much more rarely during boss fight, and trash mobs during AOE tanking cannot crush. It does not harm the threat of paladins, as they can still use Holy Shield skill anyway even if the extra blocking is not needed. Stated in reverse, because Holy Shield is still needed for threat purposes, the stat sacrifices required for passive uncrushability are not attractive for tankadins.