"Easiest" is a very subjective term for anything. I've heard many people say they like the business maths module because they found it very applicable and, as a result, easy. However, I know my school didn't choose it because they thought it was the hardest module on offer.
However, as for modules most RELATABLE to methods specialist:
Matrices (however, this module is quite different - but you're doing it with your school anyway, so not an issue)
Graphs and Relations (this steps beyond methods, though - I suggest going through your textbook for this module so you cover it all)
Trigonometry/Geometry (basically, everything you need for specialist covers it, plus bearings)
I would say that if you do want to learn a new module for the end of year exam, do it AFTER you've done the module you don't care about. So, if you want to replace business with graphs, do this after your business SAC and you've got the results. This means you still have to put in a good effort for it, instead of thinking, "well, I'm not going to do well on it anyway, so I'll just learn graphs". Who knows, you might find after you get your results that it's worth continuing to do the business module.
Also, ask your school for their opinion - at the end of the day, they can't change what modules you do in the exam, but they might have stories/resources to help you with the module you want to do instead, and so might be able to help you out.