Okay, so my Duelist died from a health overflow, I guess... lol
I don't know what is the maximum number it can reach, but I have 10x Dancer traits, all with 2x attack (except the 0/2 basic one), and the Duelist died in the first attack cycle. It started with 223 health and I don't think the Dancers attacks were over when it died.
I kept playing and reached depth 500. Decided to end the run there because I was essentially infinite and also the lag was quite bad.
Tried to rebuild a Duelist to see if the big health was the issue, but one with 144 base health that attacked 24 times also died due to the overflow or whatever. I noticed that when you start seeing decimals (above E+07) then the operations are not quite correct. 15 attack transforms into 20 health, then into 23 attack, 32 health... I'm guessing that is the same problem that makes the Duelist die sometimes (sometimes it dies and then the next combat it gets +2 health and then doesn't die for a while).