There's a coupon code event hosted by StarPlay.game that's super easy to do and gives you 2 SSS summons and 500 Rubies! Be quick, its only available to the first 100,000 people! Here's how to do it.
Click on this link here to get taken to the website. Click on the yellow button to login to your Kakao account, and once the page refreshes click on the yellow button again to get your code!
Note: The code is unique to your Kakao account and cannot be shared with other accounts. This has to be done on a PC, using a mobile browser will not work.
To redeem the code, login to the game on Android (sorry iOS) and enter the shop. At the bottom right there's a button that opens an input box. Input the code there, and press the button on the right. If successful, you'll see a success message and the rewards will be sent immediately to your inbox. You have 7 days to redeem them before they expire. Enjoy!