Losing all your cards is not intentional - that's a bug. Sorry for that, but thank you for pointing it out, we'll make sure to fix it!
At the moment, enemy AI picks a random move from a list, and those moves can do variable damage from a range. It's weighted to try and be as fair as possible, but it still leaves room for bad RNG unfortunately and we didn't have the time to make it more precise. Sorry you weren't able to finish, but thank you for playing!
Edit: So I've now discovered the Siren is mistakenly set to attack when she should be shielding. That's another bug and is likely what made this fight difficult. Very sorry!