The Feinting in Combat action of the Bluff skill is another way to get an opponent denied their Dexterity bonus to AC:
You can also use Bluff to mislead an opponent in melee combat (so that it can’t dodge your next attack effectively). To feint, make a Bluff check opposed by your target’s Sense Motive check, but in this case, the target may add its base attack bonus to the roll along with any other applicable modifiers.
If your Bluff check result exceeds this special Sense Motive check result, your target is denied its Dexterity bonus to AC (if any) for the next melee attack you make against it. This attack must be made on or before your next turn.
Condition-wise, blinded, stunned, and helpless creatures also lose their Dexterity bonus to AC. Other characters can also help you, since grappling also denies the Dexterity bonus to AC for creatures not in the grapple.
Thematically, the ninja class is designed to engage in a surprise attack and kill or seriously wound the opponents before they can react. Their sudden strike class feature expected to be a little more difficult to pull off than the rogue's sneak attack, but they are given other powers to compensate.