For 9 star, im not sure what weapons a good idea. But technique should be the same.
Go for one leg at a time. Once one leg turns red, move on to the next. Once all 4 are red, repeat until all are bright red. If at any point it falls, go for its face. Once all 4 legs are bright red and you deal some more damage after, itll fall, and itll stay down for a while. Use this chance to go for the face, and maybe bomb it there if you can.
Once it gets back up, repeat the process.
For bowgunners, cannons and ballistas are more useful as well. They deal a fixed amount of damage which is a fair bit of damage to boot. Go for its face with these. When yer down with it, go for the legs again. I hear that if yer on a bridge, and it does its acid shot thing, you can jump on its back and Anti Dragon Bomb it there, which is supposed to do some heavy damage.