How hard is it to get a 7 in IB HL Biology?

Hello, I think IB biology is fairly difficult to obtain a 7 in. But if you truly enjoy biology and and is willing to work for the 7, I think it is very possible you'll achieve a 7.
But another thing to consider is the time aspect. Biology takes up a lot of time, and I've had friends who said they regretted taking HL biology even though they didn't require it. It causes this thing where though you don't need biology, because you want a better overall score you spend time on it, losing time spent on other more important subjects. I took both economics and biology HL, and I do think economics (in the case it relates more to your major), is a better choice.
Hope this helped!