Here is what I do on my two phones when playing dungeon quest.
First i log in my email then download the save game.
Here is the tricky part when i play.
When you are going the campaign, first click the campaign button (1) once then you will experience a short lag on the game.
Then click the campaign again, this will put you in the character picking.
Before you start the game, shutdown your wifi or data or any connections in the internet you have. Then click the start of the campaign. After you enter the game, return to main menu then close the dungeon quest app, as in close it in running on the phone.
Start the application again.
You can play the game offline with no crashes.
(This is in my case PVE)
For pvp, after downloading the save game, click the battle arena once, will also lag a bit then click again, try to have a few seconds gap when click the battle arena again (If you still not able to enter in the first try). Then try to click the ranked match just once, trying to click the ranked match multiple times just put me in connection time out and will never connect to a match.
(This is i think a year or 2 since i havent pvp again until now)
This solves mostly of my crashes I experience before.
Just trying to help.
Hope you understand xD