Soul Crushes are special combo abilities that get executed once a character and your party has completed a nearly flawless combination of attacks. This happens only if you execute the perfect buttons / attacks during a battle.

Alicia
Nibelung Valesti 1, Nibelung Valesti 2

Rufus
Blinding Blaze 1, Blinding Blaze 2

Dylan
Violent Rage

Leone
Sublime Horn

Arngrim
Final Blast

Hrist, Silmeria, Lenneth
Nibelung Valesti 2

Brahms
Bloody Curse

Valkyrie
Nibelung Valesti 3

Freya
Ether Strike

All Non-story 'Light Warrior' Einherjar
Whirling Ripper

All Non-story 'Heavy Warrior' Einherjar
Soul Evocation

All Non-story 'Archer' Einherjar
Pulverising Storm

Mihtra, Ageis
Tidal Wave

Alm, Farant, Seluvia
Gravity Blessing

Woltar, Khanon, Masato
Maleficent Harm

Psoron, Xehnon, Lezard
Animate Earth