I have successfully interviewed at MBBs, Tier-2, and boutique consulting firms over the last 4 years. During my time at BCG, I was also involved in recruiting activities (resume review, interviews, etc).
I believe casing is a skill set that everyone can be taught (assuming the candidate can think intuitively and do arithmetic). But with any new skills, you need to make sure concepts are right before jumping and trying to tackle MBB cases. If the foundation is not solid, you'll make the same errors regardless of the number of cases you do.
Over the last 18 months, I've been freelancing as a coach and helped candidates secure McKinsey, BCG, Bain, Bain Capital, Strategy&, and Oliver Wyman offers.