You need enough preparation to understand the format and eliminate avoidable mistakes, but online assessments should not consume your entire interview-prep schedule.
For Bain specifically, use the relevant format as a reference. The Bain SOVA guide is useful when that is the assessment your office uses.
With ten days, start with a diagnostic attempt, identify the weak question types, then practice those under time pressure. Finish with one or two full realistic simulations.
Continue case prep in parallel if interviews may follow quickly. The assessment is one gate, not the entire recruiting process.