Playing Classic get all the way to level 99. Note, even though it *feels* like infinite attempts at this, you don't, so conserve your lives. Once you're done with level 99, the robot will come on and tell you you have to face some C.H.E. boss. This is, by far, the hardest level because (a) you don't powerups like lasers and (b) some of the bricks keep coming back. With (a) and (b) in mind, it's a pretty tough level. I just kept plugging away at the blocks in front of the center cluster of bricks. It looks like the objective is to get to the protected, central bricks.
Once you kill the middle bricks, cheevo pops!