i just finished that quest tonight with my friend. i used the titernia and he used the valkyrie heart. i attacked his head and he shop from a distance. it is also good to bomb his head when he enters a new area. we blasted off his horn when he was coming into area 2. 2 large barrel bombs each would be enough to break his horn. just set up all 4 in a small bunch and have your friend shoot it when you see his head over the bombs.
then just attack his head while he is walking and have your friend shoot at him constantly. we used both anti-dragon bombs at the same time. and then in area 5 we used the dragonator, we weren't able to use the cannon though.
i hope this helps.