My guesstimate for hardest to make the good grades would be
Chicago, Hopkins, MIT, Berkeley/UCLA
Princeton(?), Cornell, Vandy
I tend not to rank or compare Wustl in these discussions, too biased, better off asking someone like Wedgie that is more objective. Am not really familiar with premed weedout reputations of Notre Dame, Georgetown, CMU, or any but a handful of LACs. Princeton gets a (?) because of their recent removal of anti-inflation policy, may drop off the list if it eases up a bit to be more like HYS.