A harder shot on Houdini is one that shouldn't be, which is simply hitting it onto the stage to start a mode. The design on that shot (one of the most important in the game) simply has to be a mistake. Your only hope is to shoot the ball in that general direction and hope that one of the pops or bumpers knock it in.

Also, Iron Man on Avengers is a really tough shot to hit clean. Another shot on the game, which simply requires a lucky bounce is the Hulk saucer. It's another bad design. It's just too shallow.