I believe good advice can only be given if your priorities are clear. So in the following, I will just assume that MBB is your first priority (since this is what I am focusing on with my coaching). A well-tested and successful sequence can look like this:
1. Concentrate on proper preparation with excellent case partners/experts
2. Make sure your written applications (CV, cover letters, referrals) are on point!
3. Once you feel really comfortable on fit, cases and your application, apply to all Tier2 firms
4. Continue practicing cases!
5. Schedule interview days with the Tier 2 firms
6. Once T2 interviews are scheduled, apply to MBB
7. Finalize and polish your personal fit answers
8. Go through T2 interviews and get used to the setting (if you focused your preparation on MBB, you will realize that interview formats and cases might be a bit different at T2 firms, but don't get irritated by that fact). Try to get at least one offer (this is a great psychological help!)
9. Schedule interview days with the MBB firms that invited you
10. Continue practicing cases!
11. Nail interviews with MBB firms and celebrate your offer(s)!