This verse makes me laugh because it reminds me of Sancho Panza's line from Don Quixote, "It doesn't matter if the rock hits the pitcher or the pitcher hits to rock, it's going to be bad for the pitcher." It too is a play on words, discussed in the parallel verse at Matthew 21:44., And it contains a mystery word: one used nowhere else in ancient Greek except these two verses.

Let us go through the verse's vocabulary, which has a couple of small differences from Matthew.

The word translated as "whosoever" is the Greek adjective meaning "all", "the whole", "every," and similar ideas. When it is used as a noun, we would say "everything." As an adverb, it means "in every way", "on every side," and "altogether."  This Greek word does not appear in Matthew though it also has a "whosovever" that it takes from the form of the following verb. Since this adjective refers to a singular subject, we would use either "each" or "every". 

"Shall fall " is translated from a Greek word that means "to fall" and "to fall down." It is the root word for dozens of Greek terms involving moving from a higher state to a lower one. Like our word "to fall" it has a number of special meanings including "to fall into a given class", "to prostrate", "to fall from power", "to perish," and so on. Here, it is in the form of a noun "the one who falls."

The word translated as "on" means "on"against," and "before." The meaning that works best here is "before" meaning both "in front of" and "before" in time.

The play on words here uses the various meanings of "fall" and "on," and "before." The stone here is the "head of the corner," which has to meanings, the capstone, which tops and protects the wall and the head of community leaders (see Matthew 21:42 for more on ths play on words).

The word translated as "that" is an adjective that highlights its noun as being in a specific place or time from a word that means "there." A different word with a meaning closer to "this" is used in Matthew. 

The Greek word translated as "stone" means "a stone", "stone as a substance," and various specific types of stones, such as touchstones and altar stones. It refers to the rejected stone that become the "head of the corner" in the previous verse ( Luke 20:17).

Now we come to the part that we cannot understand perfect.

While Biblical translation sources define the verb translated as "shall be broken" as meaning "to crush together," there is no obvious way that we can establish that meaning from Greek. The word used doesn't appear elsewhere in ancient Greek. It uses a prefix that means "together" but the root word is a mystery. It too is not used anywhere else in ancient Greek. In modern Greek, the word here means "to be sorry" and its root means "to be afraid". However, I hesitate to use modern definitions because they most likely come from the Bible. The translation in the KJV may from the word used in the Latin Vulgate, confringetur, that means "to shatter," "to break friendship," and "to dissipate." It sense is to "break togetherness."

The Greek word translated as "but" joins phrases in an adversarial way. Since it always (except here) falls in the second position, translating it as "however" often captures its feeling better. Here, it falls in the third position in the phase.

The word translated as "on" means "upon", "against", "before", "by" or "on."

The word translated as "whomsoever" is a demonstrative pronoun, but it often acts as a pronoun, especially a connective pronoun introducing a dependent clause.

There is an untranslated word here that indicate the possibility of something happening.

"It shall...fall" is translated from a Greek word that means "to fall" and "to fall down." The same word used in a noun form above. Here it is a verb. It is the root word for dozens of Greek terms involving moving from a higher state to a lower one. Like our word "to fall" it has a number of special meanings including "to fall into a given class", "to prostrate", "to fall from power", "to perish," and so on. Here, it is in the form which indicates someone acting on themselves, so "lower themselves down."

"Grind to powder" is from a verb that means "to winnow" that is, separate chaff from grain by throwing up up in the air. It also is used as a metaphor for "to scatter like chaff."