Is there any app for reduce battery draining for ios 12?
No, there are no apps that generally lower power consumption on the iPhone.

You can enable power save mode in the Control Center to reduce power draw by background services, and you can uninstall or limit apps that use background services to stop doing that.

However it sounds like you only have the large power draw when you're actually playing the game. In that case it is the game using the power, and there's really not much to do about it. You can check the game for settings that allow reduced power usage, or write a bug report / complaint to the game developers.