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Dual Crush skills are a sort of 'overdrive' move used within Portrait of Ruin. These are often very useful if there are multiple enemies and you wish to deal a large amount of damage to them, if not kill them outright. Surprisingly, some of the 'weaker' Dual Crush skills remain useful even in the later stages of the game.

Dual Crush Name	Location
Rush	City of Haze
Holy Lightning	City of Haze
Axe Bomber	Great Stairway
1000 Blades	Sandy Grave
Volcano	Nation of Fools
Meteor	Forgotten City
Grand Cruz	Complete Quest XXX: Master the Holy Power. Wind will give you this Dual Crush Skill upon completion.
Divine Storm	Dark Academy
Dark Gate	Forest of Doom
Greatest Five	Clear the Nest of Evil. It is in a chamber beyond the final boss.