You said there wasn't an easy option, when just spamming ember really is the easy option tbh. It takes longer than oneshotting with bubble/vinewhip, but you'll still finish the fight super easily.I know, I... said that in my post.
The scariest thing it can happen is screech bind, but if you get a lot of screeches you should swap anyways, and onix isnt know for its good attack either.
Its less bruteforcing and more so just spamming A until its done