Knockin' on Heaven's Door
Be aware of two main concerns. Firstly Crushing Blow - CB - is a very different kind of peanut and much unlike Critical Strike - CS - or Deadly Strike. The effects of both properties don't have much in common and don't interfere with each other. CB is simply another kind of damage, being applied before other damage sources.
CB deliverage is indicated by a purpleish little puff animation over enemy's head, very similar to Barbarian's Bash animation. In environment with lots of concurring animations I've found the CB animation been skipped lots of the time.
Secondly there is much toying around with formulas referring to player count, and discussions about the term 'scaling' and if you may or must not call CB damage scaling.
CB damage substracts 25% of a normal monster's actual Singleplayer mode life, if that monster has no physical resistance.
Even in Multiplayer mode Singleplayer hitpoints of the monster are referred to, and actual life, not maximum.
More detailed musings about CB damage are variations on that theme, so stop by a moment, chew some peanuts and meditate about that statement.
CB needs a physical attack (blowing, throwing or shooting) with a successful Chance to Hit check. Spells, even those dealing physical damage, won't trigger a Crushing Blow.
You may achieve a better chance for doing CBs with more appropriate items equipped, the chances simply add. However, a chance of more than 100% is not better than a 100% chance, 100% means every (successful) hit will deliver a CB. There's rumour about a limit of 95%, but this is simply due to the fact, that most attacks are subject to common CtH check. CB chance itself is not limited.
Amazon's bow skill Multiple Shot will deliver CBs only with the two middle arrows of a spread.
CB% = 100 / (2 * (Ppl + 1)) %
CB%: percentage life loss of actual life Ppl: player count
Versus Champions and Bosses the resulting damage is cut in half.
Also CB damage delivered by a ranged attack is cut in half.
Versus Objects CB damage causes 1/10 'life' loss, again halfed if delivered with a ranged attack.
So if a boss suffers CB delivered by a ranged attack he'll loose 1/16 of his Singleplayer mode life. This doesn't look impressive, but counts a lot especially against Act Bosses and other monsters with a huge amount of hitpoints.
Since 1.10 Foul Crow Nests can spawn as random bosses. Don't rely on a bow in combination with CB to defeat them quickly.
Versus hostile players CB will cause 1/10 life loss in melee, 1/20 with ranged attacks.
A german table made by black_spy can be found on d2wissen.
Positive physical resistance diminishs CB damage, thus a Physical Immune monster doesn't suffer CB damage. However, a negative physical resistance does nothing to enhance CB damage. Paladin's Sanctuary aura will ignore undeads' physical resistance, but this is not taken into account regarding Crushing Blow.
CB delivered by a hireling is no subject to hireling specific penalties, which differs to regular damage order.
Versus hostile players there is no further penalty than the aforesaid cut-in-tenth one..
Blunderbores, Gorebellies, Urdars and all subspecies hereof have a chance of 15% in Nightmare and 25% in Hell to do a CB.
CB in Patch 1.09
In 1.09 CB damage was applied after all other damage. It worked based on actual Multiplayer mode life.
Versus normal monsters CB substracted 25%, versus any other unit 10%. Ranged attacks cut the result in half.
Each arrow of one Multiple Shot spread could deliver CB back then.
Also CB was carried in Exploding, Freezing and Immolation Arrow's splash damage. This doesn't work anymore for 1.1x. CB is just checked for the main arrow, the splashes only carry corresponding elemental damage from equipment.
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