You will face this boss during Lucia's campaign at the end of Mission 7. I would recommend using the Bow Gun which can be found in Mission 6 using Storm101's solution:
"...right at the end of mission 6 and is in the underwater section in the area nearly opposite from the door forward. Careful you dont go through the door as it will end the mission and you won't be able to get it."
All you need to do is wait for the boss to become visible so you can damage him. I just moved up and down to dodge his attacks which consist of him charging you or hitting you with his tail. I kept firing the Bow Gun constantly and was able to beat him without taking much damage. The boss shouldn't give you much trouble since you can only get the achievement on Normal difficulty as Hard isn't available until you beat the game with both Dante and Lucia.