I learned a trick from a friend that makes it easier to kill grass. She used an old carpet to cover the ground and left it there for a few weeks. Thick cardboard will probably work too. Without light the grass died and was easier to hoe up and scrape off, then she tilled and watered. If any weeds came up, they were pulled. Then the soil was amended and the garden planted. Roof flashing was placed around the edge of the garden plot to keep surrounding grass from creeping back in.
It is a slower process, but since August is not a great planting month anyway, it required the least work.
It is still a good month to solarize though. After taking off the grass, you could probably put down the 4 mil clear plastic and solarize, which should help kill the weeds and nematodes that are in the first 4-6 inches of the soil.
Happy gardening in Hawaii. Gardens are where people grow.