Initially, both the player and the monster have 100% health. Player can use attack, heal and use special attack on every third round and monster attacks back the player every time he attacks or use heal/special attack. Player can surrender, too. There is a battle log that reports who attacked, what type of attack was used and how much health was taken.
You can test it and play it https://ivanovskiviktor-monstergame.netlify.app/ :)