For this one, you want quite a bit of money. Hero upgrades are the little square boxes next to the level up button. Start with Cid, the Helpful Adventurer and level him to 250, then buy the seven upgrades he has. Move onto Treebeast. Level them up to 175 and buy the five upgrades he has. For Ivan, the Drunken Brawler, it's level 125 and six upgrades. Brittany, Beach Princess should be levelled up to 75, and then buy the four she has. Once you've upgraded him to level 100, you can buy his five upgrades. The achievement will click after you've bought his third upgrade.
Note: This also stacks with Level Adventures Up x amount of times achievements.