What is the correct punctuation/grammar for this sentence?

Is this sentence correct punctuationally:

Anyone who has tried it at least once, knows that...

Also, I googled it to find exact matches, but got only 4 results.

Here's the context:

• It's hard to do it fast. Anyone who has tried it at least once, know that it's a complex task that requires a lot of focus...

So, how to convey this idea properly, with correct grammar and punctuation?

Answer

There are a couple of things at play here: people like to put commas in where one would pause in a sentence, but that is not the reason to put in a comma.

The other thing going on, more to your point, is identifying the parenthetical part of the sentence. If we look at it that way, we can see different ways to punctuate it. "Parentheticals" are expressions that can be removed from the sentence, yet the rest of the sentence still makes sense. (In my teaching, I call them "oh, by the way" phrases. In theater, they might be thought of as audience asides.)

"Anyone who has tried it at least once, knows that...."

Is the basic sentence "Anyone knows that..."? Then the parenthetical part is "who has tried it at least once." I don't think this is right, because the original sentence then implies that those who have not tried it at all would still know whatever it is. If it were true, however, the sentence would be written this way:

Anyone, who has tried it at least once, knows that....

Is the basic sentence "Anyone who has tried it knows that..."? Then the parenthetical would be "at least once." The sentence would be written this way:
Anyone who has tried it, at least once, knows that....
This could make sense. In this case, the compound subject would be "anyone who has tried it."

Another choice would be if we are saying specifically that the compound subject is "anyone who has tried it at least once." In this case there would be no comma at all. You don't put a comma between a noun and its verb, a subject and its predicate (except for "oh, by the way" expressions) even if the subject is wordy. In this case, the sentence would be written like this:
Anyone who has tried it at least once knows that punctuating a sentence isn't easy.

A final thought: do you really need the "at least once"? In our speaking, we tend to repeat things that are self-explanatory. (Such things are called "pleonasms," if you're into obscure words.) For example, someone on a television show I'm watching just said something was a "shocking surprise." She could have just said it was a shock. If it's a shock, it probably was surprising. In writing, we try to be more succinct. If you've tried something, you tried it at least once (the first time).