House Flipper

A moderately efficient way to get this achievement would be to repeatedly purchase "Abandoned House" and remove all of the weeds from the yard every time. There are around 200 weeds on the property to remove, and you shouldn't have to purchase the house more than six times to max out every skill tree (especially if you make weeds visible on the minimap as soon as possible).
Be warned, though, that unless you have the negotiation skill tree maxed out or you clean up the house a bit, you're going to lose around $6,000 each time you sell the property. If you have every other achievement up to this point, though, this shouldn't be much of an issue.

If you decide to buy this DLC then you would think this would be a grind but its really not.

Play the missions normally and when you unlock the Petrol Lawn Mower that's when you can speed this up, when doing a mission that requires to remove weeds and mow the lawn, just mow the weeds and it counts towards your skill point challenges. NOTE: You cant mow all weeds but 90% can be.

After you have unlocked the Mower another good place like mentioned in another guide was to buy the *Abandoned House* - there is a load of weeds in this lot and 1 minute with the lawn mower around the house should net you a handful of progress, sell the house and repeat. You will lose around 7,000 currency profit doing this but if your like me, I did this after the main base game and had 1.5 mil by the time I got the DLC