Its basically acetone to melt the tip, save your money, you need to thin the whole tip any way to ensure the final nail is not bulky so spend the time perfecting your blending.
I buff upwards (abrasive on the tip and stroke upwards from tip towards cuticle direction) then you don't have to worry too much about hitting natural nail as your abrasive only touches the tip. Once you've thinned the whole tip blending the line is easier. I then hold my abrasive horizontally and again only on the tip, and stroke right to left to blend out the line.
It is practise that makes it easier but also you need to slow down and consider what you are doing....if you are filing but nothing is happening then look at your abrasive position....is it actually buffing the area you want? If not change position and try again...if its still not change again...once you master the skill it is pretty easy