How to revenge a grocery store?

Let's say hypothetically some naive teen went to go to work for the first time without having a contract and racked up 500h or so of work-time. However only received about 1/8 of the pay, were promised to get it later.

The boss then fired them, claiming that all days called in sick were not actually excused. The teen does not have any copies of doctors notes and the doctor doesn't have them either because the sick days were months ago.

The boss says they don't know anything about "pay" and refuse to pay any money. And legally speaking that teen cannot sue because technically they worked there illegally because they never signed a contract.

This is not in the USA btw,

What could that teen do to revenge himself?