I’m having the same issue started yesterday and lost energy and boss key. Deleted and downloaded the game 3 times and still doing it. Maybe the developers need to make a new patch or something to help with this . ASAP 

I can’t keep the game running for more than two minutes without it crashing. However if I attempt a battle it crashes immediately. I’ve been playing for ages and it’s really pissed me off

Try on a different device, check your internet