To get a band six you just need to: do your home work, study before tests, manage time, be good at school allready (ie, competent), and when you're stressed, JUST RELAX!
However, with the Excel Science Handbook, it did help me, and not in the least way that it reduced my stress and anxiety. When I started studying for the SC I wanted my notes to be organised and comprehensive and the notes in the Handbook were just that. Most of the things in there I already knew off-by-heart, though, as with biology (not my thing), it covered what I needed to know for each topic. It is very usable. Even if I didn't need to revise any more, skimming through it quelled any irrational nerves.
So yes. I'd advise you get it. Its not expensive. It helps. You should get it. Even if only to reduce your nerves. Just buy it.
And relax.