Appreciated! Yeah too many summons can break the game, and we haven't figure out how to fix it yet. And instant resolve (or even faster speed) is on the agenda; it just takes a lot of work since the game is originally coded assuming that there'll always be a delay between each action!
That's likely integer overflow. So when the duelist's life exceed 4,294,967,295 it'll become -4,294,967,295 instead and instantly killing duelist. It's definitely not intended but it's a interesting and somewhat amusing interaction, and it'll be tons of trouble to fix so we decided to leave it for now!