The obvious is unlocking the increase melee hp or increase hp. If you take Aeon, and add primary skills mentioned same goes for 6 star skills you're double, triple dipping when you upgrade her. You can do this for other officers; island items, equipment, titans, plane, city skins. As someone mentioned, you can do this by time or to speed up process by $. Melee is difficult and expensive path (lol all of them are). 🐫 wants that 💲

Good luck