MBBS in my opinion isnt tough at all. Most people read those crap books like Harsh Mohan and AK Jain ffs. All this says is people don't try and make sense of the material and just mug to get good marks in uni exams. Ask those toppers any Physio and Biochem or Path 2 years down the line and theyll know jack. I feel if you read for an international exam instead of these rubbish MBBS exams you'll definitely benefit far more as you'll find solving clinical problems easier, USMLE is basically a clinical exam after all. From experience I can only study from USMLE resources. (Costanzo eg). All the other alternatives just seem like a waste of time to me.