it will take you sometime before being able to complete events completely. Usually when they show up they are divided in 3 battles, an easy one, a medium and a hard one. I’m only level 22 at the moment, but I still struggle to complete the medium level ones, so it takes a lot of leveling up.
Everyday there are different missions that show up to unlock additional card slots for your characters. I suggest you complete them if you want stronger characters. Also, never forget to get your free card everyday. Weapons give a good advantage. The more you have the better.
and that’s pretty much all I can think of at the moment, good luck to you!