Paganini is a demon violinist that can be found in the abyss by looking for a room with a violin icon on it, in the map. When you enter a room like this as you explore you should be able to hear a violin playing and see Paganini.
When you interact with him you will enter a dialogue where you have a decision to make, choose to answer yes and he will drop an item for you. From there just continue on your adventure normally and he will spawn randomly down the road and attack you, simply defeat him when he attacks to unlock the achievement.
When the attack starts it will most likely include 1-2 other random enemies which are usually more of a nuisance than Paganini himself. Paganini's attacks usually involve him playing his violin, which gives you plenty of time to attack and interrupt him.