TY the Tasmanian Tiger

This isn't as daunting as it sounds. 50,000 opals are pretty much a requirement to buy all of the rangs in the store, so if you are going for the Rangtastic achievement without buying the rangs for cash, then you will get this during that. If, by the end of the game you do not have this achievement, you can replay any level via the main menu and grind out opals. In order to get the most opals, make sure to get the opal bags in the levels whenever you see them, and collect the opals from defeating enemies before hey disappear. Make it a point to collect every opal available, including opals in crates. Not too difficult, albeit a little grindy towards the end of the game. Personally, I found that grinding the second to last level was the easiest. Switch to the Cryptorang, and you will see a platform right in front of you with two bags worth a total of 200 opals. Grab them, then exit the level. This brings you back to the menu where you can navigate back to that level and rinse/repeat.