The legend of Zelda: A Link to the Past Trinexx boss battle

To defeat Trinexx, you will need both the fire rod and the ice rod.

Trinexx has three heads: the ice head which is blue; the fire head which red; and the stone head which, as you may if guessed, is the colour of stone. When it shoots ice at you or attacks you with it's ice head at all, use the fire rod to attack it. For the red head, do the reverse of the blue head. When it attacks you with it's stone head, slash it with your sword.

Repeat this and soon Trinexx will be defeated.