Brave Frontier was made to work with more modern devices with at least 1GB RAM/memory. It's advised to update to a better device but you can try the following:
1) Tap home
2) Tap menu
3) Tap settings
4) Turn off volume and battle effects
5) Restart device
6) Turn off every other app that isn't needed
7) Play and repeat the process every 30 minutes to 1 hour of gameplay, depending on how quickly the device slows down.
There is still no guarantee of a smooth or uninterrupted gameplay experience even if you follow these steps if you are using a device that is below the minimum spec requirements. If you want to know the minimum requirements please click Here.