The lower star card will win a tie. If the stars are the same, I think it's just random.
If you play non random and the attack is the same the lower star count of the dueling cards wins. In the event both cards have the same attack and the same star count the attacking card is the victor.
This is assuming you play non random.
If you are in a random room, then a little bad luck... but thats still a pretty small sample.
if you are in a non-random room... it seems to me that the lower leveled character always wins. if characters are the same level, I haven't figured that out yet (i assume then its either random 50/50 or that the attacking or defending character always wins)
In random rooms, whoever has the highest pill has a greater chance of winning even if the other player has a higher AP.
In non-random rooms, in the event when both has the same AP, star count comes first. If your card is let's say 2* and the other is at 5*, you win. But if both has the same star count and the same AP, the attacking player wins the round.
@ shadowcouncil : that was the answer i waited 4. thx that's explained it