How to easily defeat any argument for God

[Belacqua]

I don't agree it ultimately boils down to tastes and individual preferences but rather there is something that makes sense about defining bad as that which causes harm, for example. From an evolutionary perspective and individual perspective, it makes sense to see it this way.

Right. It's a definition we make. And there is "something" that "makes sense" about this. 
That's true, but it certainly doesn't assert that our definition is true or unchangeable. 

What I mean is, if you look at the two statements below:

Harm is bad.

Harm is good.

One of them is almost true by definition. 

Yes, I'd say that the word "harm" just means "do bad to." That's what the word means. 
Now we still have to prove that doing bad to people is something we ought not do. 
If it's just true, then it's transcendent. If it's not just true, then it's not true. 

The other no rational person would agree can make sense. It's certainly not practical at least. Imagine a world in which the standard is harm is good.

Yes, in real life, things get really complicated but at the core the first statement just seems self-evident.

Probably no rational person would agree that doing harm is something we ought to do. But can it be proven? Is consensus enough? 

"It's not practical" is not a very good ethical argument, I think. "I refrain from killing because other methods of getting my desires are more efficient" is not something I'd argue.

So I think the argument hasn't progressed. We can take it as axiomatic that wellbeing is a good goal, but we can't prove it. If "wellbeing is a good thing to go for" is true (not a preference) but not provable, then what is it?