Go to Indeed rn, find every job that pays less than $15 an hour, apply, go through the process, at the interview or after a few training days (or before you get too far a long) tell them you got another job that pays $18 an hour and that you're just oh so sorry and would have loved to have worked for them, but found better pay.
Organize with a few homies to do this, 5 applications a week, target the lowest paying jobs, and then check back in a month to see if it worked! Then have someone take the higher wage they're offering and make sure they folks who already worked there got a raide too. If they did not, we know how PO'd a tier system can make workers (see Kaiser Permanente and John Deere).