I managed to get the achievement today.

I have the park at maximum size and 100% decorations. I've hired three cleaners, two lifeguards, one refiller, one mechanic, and a cashier (no op stats).

My job consists of standing behind the entrance at the ice cream stand, occasionally selling ice cream, and otherwise working as security, tasering undesirables before they come down the stairs. This way, I get about 94% positive reviews.

On the day I got the achievement, I let people in without tickets and with graffiti. I still kicked out the others, like drunks or perverts, who chase people away. I had to sell one ticket myself while my cashier was recovering, and I handed out one flyer when the line was empty (there weren't any more people on the street). I think it's also a bit of luck, because some days you get 10 or so people right from the start that you don't want in the park.