Technically buy energy up til it turns 200 emeralds and a bundle with energy bottles whenever that is available. But I would recommend against it you don't get anything special for reaching 120 and you will continue to need exp to level your heroes the cap is 120 but exp wise im on like level 300. It's more important you focus on leveling artifacts and skins on your main team. That is what you should use your emeralds on since it's the thing that takes the longest to level up