When you hit an opponent affected by a fear effect (s)he becomes flat-footed to you until the end of your next turn. If you are full attacking, either that round or the following round and make at least 1 unarmed strike, you can make 2 additional unarmed strikes against that target.
How does the Shattered Defenses feat work with Medusa's Wrath?
My best guess is:
Your first attack that hits a shaken opponent causes him to be flat-footed to your attacks until the end of your next turn.
If you then make an unarmed strike as part of a full attack, you get two more unarmed strikes at full BAB.
I don't see why you couldn't combine the two. For example, and assuming the opponent is already shaken:
Attack 1: Hit, opponent is now flat-footed to your attacks due to Shatter Defenses
Attack 2: Unarmed strike, activates Medusa's Wrath.
Finish rest of full attack, if applicable, and two Medusa attacks both at full BAB
You need at least two attacks, even if the first attack is also an unarmed strike, because Medusa's Wrath only activates if your foe is flat-footed, and he's not flat footed until after the Shatter attack hits.
Wouldn't the first attack both cause the enemy to be flat-footed, AND count as activation for Medusa's Wrath? According to the feat description:
"Whenever you use the full-attack action and make at least one unarmed strike, you can make two additional unarmed strikes at your highest base attack bonus. These bonus attacks must be made against a dazed, flat-footed, paralyzed, staggered, stunned, or unconscious foe."
I honestly see no contradiction. The attack triggering the feat an unarmed attack needs to be a part of a full-attack (Flurry for example), but it does not have to be against a flat-footed enemy. It just has to be an unarmed attack. Then you can dash out two extra attacks against a flat-footed foe. Note: this foe does not necesserily need to be the same enemy attacked with the activation punch.
No matter what the two feats synergize insanely well and if you can combine them with Enforcer/Cornugon Smash and Intimidation Prowess. I use this for my Martial Artist Monk archetype and I'm a more potent damage-dealer than the two-handed fighter.