This method involves editing game files, backup before doing so.
In the root directory of the game you will find a folder called "Packs", in here you will find a zip file called "Pak0.zip". Open this with winrar (or any other program that reads zip files). In here you will find all the files you need to edit. Here are some edits that i found quite usefull:
Make vehicles (almost) invincible:
Inside the Pak0.zip go to the following directory:
Scripts/Cars. Here you will find an xml file for each of the games vehicle. For example the taxi (taxi.xml). Unzip the file somewhere and open it with any text editor (like notepad). Below the file you will find the lines:
Change the health value from 350 to anything you want, something like 100000 would do nice. Now save the file, and put it back in the Pak0.zip You can do this for all the vehicles.
Complete each requirement to receive the achievements.