This solution is for the current version of the game (and according to EA the last version there will be) Windows Phone store version: 188.8.131.52, Game's About page version: 2.8.12
For this achievement you need to accumulate 30 million town value in total. You can check your town value from the right side of your stat bar (blue figure).
The fastest way I've found to grind town value up without doing micro-transactions is to use the fact that whenever preteens level up in school they get a town value bonus. Every time a preteen gets to grade A+++ you will get 30k of town value added. Now, if you leave and enroll your preteen to elementary again their grade will be back at A++. From here it only takes three brainstorm sessions for you preteen to get to grade A+++ again netting you another 30k of town value.Big thanks to mamaani2nd for the following trick:
You can actually make this method three times faster if you make your A+++ grade preteen to brainstorm and only after that leave school and enroll again. If you keep your app open the preteen will be back at A+++ grade after only one brainstorming session! To make this trick to work you can't close or switch the app and can't change houses. Also, pausing the game and anything that will pause the game (like calendar alert) will prevent this from working. If the trick fails because of some interruption previously listed, you will be back at normal speed - meaning 1/3 of the way to A+++. To try again you need to get your preteen back to A+++.
A single brainstorm session will take 3 minutes with a 3-star study desk which costs 3LP. And 3min 30sek with a 2-star one, making it possible to get 90k added to town value every ~10 minutes with the three preteens you will need for:
In order to start this grind you will need preteens unlocked by completing various challenge tasks which will take time.
Buying various things also adds to town value and I've noticed that renewing interior walls and floors will net you 50% of the simoleons used to town value. Building houses will also give you almost 50%. If you happen to have tons of simoleons then you could ramp up the town value by buying these over and over.
I do not suggest you selling anything as that will actually lower your town value. So do not sell anything from the house you're about to demolish and rebuild if you've not gotten the two town value achievements for 12 and 30 million.