With the addition of drawn-out delve battles and the high stakes that come with them, it would be nice if battles would save to the device in case you quit the game (or the game quits itself) and wish to continue the fight at a later time. There is a retreat button for people who actually want to leave the battle, so there’s no reason quitting the game should leave the battle too.
Once you complete a room the progress is saved and you can return later to pick up from where you left off.
Unless your suggestion is regarding in-game / mid-game battle you can quickly finish the room before shutting off?
“quickly finish” isn’t exactly an option sometimes when there are 2000 hit points to burn through. Also, I’m playing on mobile, so every once in a while if I switch apps too often (texting for example), the game restarts itself and counts the battle as a loss.
The issue is, as far as I can tell, that another app overwrites the cached data from GoW.
The only solution (not ideal) is not to run too many other apps that will take up the memory and cause this 
Hey, these battle theme games are really interesting. I have also played this game, and it is true that you need to finish a room and then shut the game or else you have to begin it again. This is one of the common problems that many players face while playing this type of games, it is better to switch to PC gaming than to play the game on mobile. You can also check here and try other games similar to this like the Player unknown battlegrounds, Call of Duty World War 2, etc.