Or, you know, add such "big words" to your vocabulary when you learn their meaning. Probably a troll, but I'll continue anyway:

If the character concept is one where he or she is extremely intelligent, a bookworm, and talented in magicks; the character dialogue would reflect the traits of the character. It would not simplified into layman's terms so a potential reader who may not have certain words (like "puissant" in your example) in their vocabulary immediately understand without a bit of thought or looking up definitions. A character more along the lines of an average person would be far less likely to use such words.

Or take Jacke in the Rogue's Guild for instance. People don't understand what he means half the time because of the terms he uses (which is pretty much just thieves cant, which was like rogue slang in the 1700s-1800s). At one point Jacke says "(...)But I warn ye: he'll talk the bloody wattles off if ye let him!". "Wattles"" is another term for ears, just as "puissant" is a another term for powerful.