Getting to 80% value isn't too bad, but you'll need a bunch of money. The nice thing about the achievement is raising your stronghold's value increases your income. Just be sure not to let your chest get too full or you won't be able to get any additional income. I'd usually just head back to the villa when the chest got to about 3/4 full (you can view the current amount of money in the chest from the inventory screen in the pause menu).
When you upgrade your villa, concentrate on the building upgrades first. They offer the best value for your money and you'll earn discounts from the shops you upgrade. Once you upgrade all the shops, I'd focus on the armor and weapons collections.
Collecting feathers and codex pages will increase your value rating, too, but not by much.