The Bain test is heavy on chart interpretation with a few words, as well as numerical computation and reasoning. It is important that you are familiar with this type of question and practice consulting math on a regular basis.
My recommendation to prepare is to:
As an additional note, Bain also uses two maths tests to screen candidates in some countries (and you might want to use them as a practice). One is largely based on GMAT and includes 15 critical reasoning, data sufficiency and problem solving questions to be answered in 25 minutes. The other one is very similar to MCK PST and BCG Potential Test and contains 2 business cases to be answered in 45 mins. These tests have been used only in South America so far. You can find some examples to practice here (https://www.dropbox.com/sh/k4akdv138j7ma3d/AAAuKT3OyPWO0aZ8h2v8ZQo4a?dl=0) but I haven't heard them being used in Europe, North America, or Asia (yet).
Hope this helps,