Click the buy button to buy weapons, then select a monster to battle. Hover the mouse over a monster in the choosing screen to see what it's weak to. You start with 5 gold and can buy any green weapons. Killing monsters gains you gold and health... You cannot beat a monster with the wrong weapon, at least not at the start. Gold gain goes up as you fight tougher monsters while XP gain remains the same. (Leveling up just gains you health) This game is point-and-click and might work on mobile too, but I'm not sure if the size is right for mobile. If you have the right weapon you always kill the monster in four swings. If you use the wrong weapon you do 1 damage. You can kill the lower level (green) monsters with the wrong weapon when you're max level, but I wouldn't recommend it.

Cheers, have fun!