Moonlighting Guide (20G): Reach tier 10 in the Street Scene.Some major points I'd like to make clear:
- The only way to unlock this achievement so far is by just racing/ grinding the street scene races in Horizon.
- You won't receive street scene influence when you're joining/ doing a team adventure where they're doing street scene races, but you'll instead receive team adventure influence or whatever it's called.
- The way to level up in this game is by earning influence; so grab yourself a FE (Forza Edition) car to boost your influence. I also highly recommend you to purchase influence boosts in the skill tree with your perks.
- There's no way to easily grind your street race influence like on the road racing series. You can't do blueprints with street races so you can't race The Marathon (biggest street race) until tier 9 of the street racing series.
- Difficulty doesn't matter for your influence, you could even drive on new racer and get as much influence as someone that is racing on unbeatable.So to make it even shorter: Grab yourself a FE car, unlock nearly all the skill perks and do some street races, try to race clean and skillfull to max out your possible influence. Of course the longer the races are, the more influence you'll get, but I highly recommend you to keep the diversity in the races so it won't become tedious.
Reward for tier 17 in the street scene:
So good luck with grinding!

*Another possible solution:
Not only by doing street races when doing the events you'll receive influence for the street scene but also by doing head to head races against the AI or PvP while driving around. I haven't found anything about the influence numbers for the difficulties: easy, normal and hard. The only thing I know was when I was tier 13 in the street scene I did around 20 head to heads against the AI and got to the next tier (they were all easy opponents).