Today, I learned a new phrase "Tit for tat" from onea post on Reddit.

Using the indefinite article "a" is more correct than "one" in this context as it does not emphasize the numeric quantity of posts that you saw on Reddit, which is not important to the meaning of your sentence.

The OP said: "Tit for tat in the tech world."

Which means,: when there' are two parties are battling, if one hits then the other party will hit back within the same way.

Colon is a good choice to avoid a comma splice after "means" because it connects a dependent clause ("Which means") with an independent clause ("when there are..."). We choose "when there are" to replace "there're" because "when" is the correct conjunction and "there're" is not a contraction of "there" and "are" (this contraction doesn't exist in English). Using "when" creates a conditional clause: when this, then that. "Hits" is the correct conjugation of "hit" when using "one" as a pronoun. Finally, "in the same way" is the idiom, not "with the same way," which is a phrase you'll just need to memorize.