Download Dr Green
In Dr Green you will have to take the role of a soldier that must completely destroy the "army" that Dr Green himself managed to create over years in order to take over the city and go even further. However, you will have a hard time as there are all kinds of monsters in the area.
The monsters are pretty nicely designed, as the game features three dimensional graphics that look nice. Even if your shotgun has unlimited ammo you should still watch out, as the goblins are very dangerous as they can move very fast, especially if they come towards you.