If you are having problems making it to 20,000 coins check out the section on combat in the project description. I've copied it below.
Combat
Each hero has an attack and a defense represented by a A/D over their heads. The attack indicates how many strikes they can deal out each tick. These strikes are dealt out at random to all engaged slimes. The defense indicates how many slimes they can defend against before they start taking damage. Make sure that you always have more slimes attacking a hero than its defense.