To do the synergy attack you have to use the alt combo with max unity for example for the basic sword Y,Y,(pause),Y,Y,Y. You can tell when you can start it by having the upgrade to unlock it and when the weapon shines and sound que plays, you have to finish the button sequence or you don't get the synergy attack. In game they just tell you to do the alt combo but they don't tell you that you have to buy it for other weapons and you have to buy the accompanying upgrade in Lute's tree.




EDIT: For the last hit you can change the synergy attack to a different weapon on the last hit ie. hammer to fist or sword to bow. As long as you do the sequence (with the all the requirements) it'll work. ie. Y,Y(pause),Y,Y,X if you wanted to do start with sword