While there is a luck factor in getting both the big crystal and the big crystal, there is also a skill factor. It can be done with any level, but it is probably easier with bombinos. You need to get the letter at the exact moment of death, which is easier to predict when familiar with how long it takes for a bombino to explode than trying to figure out exactly what moment an enemy dies when it falls off the screen.
Note: the time of death of a bombino is the moment it explodes, not when your bomb hits it. I am not sure exactly what moment an enemy dies when it falls off the bottom of the screen, but I would guess a little bit after it is completely gone, and when the white light starts to appear.
There is an easier way than that though. If the last enemy actually died before it got to the bottom of the level, then getting the last letter before the enemy reaches the bottom of the level will also work. Some examples of how this happens are black bombs, the enemy falling on spikes, and a few items like


. Using this method, you actually don't want there to be any bombinos. A good level to try this out on is level 52, due to having black bombs and the last enemy is quite high, giving plenty of time to get the letter before the enemy is off the bottom of the screen.
That particular time was actually the 4th time I have done it, but the first which required very specific timing.