I obtained this achievement few days ago and was very happy, relief, contented, satisfied.. and more.. 
Now I'll give you my strategy to win this.. I am not an expert on this game.. just sharing my experiences, ok? You may try it or not.. it's all up to you..
How to obtain BulletProof Achievement?
1. You must have good values of skill points (both of attack and defense) at least double your level.
2. Do experiment about the best time to put yourself on the HitList.
Choose regular working days and do this at midnight or dawn, at least people on your time zone don't play.
3. When you're already on the HitList, if you loose by one person, quickly go to his/her profile and put him/her on the HitList too.
This will make him/her busy being attacked and stop hunting you.
4. Check your health points regularly in a very short times, whenever it gets low go to the hospital.
Don't wait until you are weaker and weaker.5. Don't you ever do any fight (attacking or robbing), save your health points.
We never know someone stronger attacking you continuously. You'll snuffed in just seconds.6. Choose your name title to lower/lowest level. Street Thug or Associate would be fine.
In this way, you'll just attract lower level mafias. They will attack you and (hopefully) you could always win and gain lots of experiences. Since they know you are unbeatable, they'll stop attacking and you may go to the hospital to cure yourself.
7. Luck factor!
This is the most important, I think. I was just so lucky that no higher levels attacking me. Been hiding perfectly there until one scum snuffed me.. so close.. 55 seconds longer! Lucky me... 
8. It would be much better if you collect The Loot Items with the Highest Att/Def.
Give one to each of your mafia.