Intelligent, yet emotionally broken and poor WWI veteran is laughed out of the room when he applies jobs at large banks during the 1920s boom, until he finds a mentor in the head of a near-bankrupt bank who gives him a job and helps him hone his trading skills before the Crash of ?29, when the hero executes a series of very complex trades, designed to take down each of the banks which underestimated him, one by one.

Great replies so far!

Focusing the story on what the main protagonist wants and how they get it is key here. A no frills way to boil the logline down is to imagine the movie trailer for your film and write 20-25 words describing it. If you read Tony's version you can almost see the trailer materializing.

Very wordy and grammatically incorrect. Does the WWI Veteran, emotional broken and poor have some significance? Do these things drive the story?
Might I suggest.
The Stock Market Crash of 1929 is on the horizon. A WWI Veteran finds a mentor in the near bankrupt head of a bank. With the help of the banker he executes a series of complex trades designed to take down the bankers that caused his friends ruin.

Nice concept, but way too wordy. Cut it back to under 25 words for a cleaner, clearer logline. I'd suggest figuring out exactly what's most important to the protagonist, as well as what's standing in his way, and using that to rework the logline.