Give more time to set blockers when opponents attack with a lot of creatures, and make the defenders easier to see.
I lost a game I had won, because I didn't have time to set my defenders, along with not being able to see who was defending who. My opponent had their cards all with trample, and I had enough to block them, but I messed up one of my blockers, because I couldn't see which creature wasn't fully blocked, and it was able to get through and end me. I think this should have 3 different things happen.
1) Give each attacker a different color, which resets after 3-4, or even just every other one. This should be enough to separate the lines easier, and make it easier to see who is defending who quickly.
2) Show how much damage will get through for creatures with trample, maybe directly below their attack damage in red. This isn't an issue in a normal game, because you have time actually count it out. But in Arena, you are expected to act quickly, so we should be given more ways to do so.
3) Either give an increase to the timer, depending on the number of attackers (something like every attacker after the third gives you an extra 3-5 seconds), or delay the timer if the other player is actively setting defenders (but this would require a way to prevent players from setting and unsetting cards as defenders in order to purposely delay the game to frustrate the opponent).