In World 8-4, go to the door at the bottommost right. Help Wario through the spikes, and get him to the water surface. Go through the door and use the "ground pound" by double jumping above the three stars then pressing "down + B". You should dive below the water surface and defeat 6 squid enemies before coming to a halt, with them revealing 3 big stars and 3 small stars. Swim up and defeat the lone squid and super jump up to a staircase. This time, double jump again and "ground pound" down to the same area again, where you'll defeat the squids there again and gain the stars once more. Repeat till you get 9,999 stars.