There's a really detailed solution at the sister site TrueAchievements:https://www.trueachievements.com/a232940/i-want-it-all-achie...
Just in case you come here first, this is sooooo much easier if you don't have natural disasters installed. If you do you're expected to get all the extra Unique Buildings and the Doomsday Monument - which involve beating Floodlands and The Dam, two seriously testing scenarios.
Not sure if there's a bug or something, but I've done this playing on a European scenario and NOT placing the Gherkin. Just make sure on your main city you put everything down.
*This note was added after the site suggested it wasn't obtainable. Well I've managed it so assume they've sorted a bug out