1. Make sure your house isn't drafty and you have good curtains to keep out the blazing sun
2. Consider replacing the main unit. I had an undersized airhandler that I replaced last year. The old one wouldn't keep up in the summer (Houston) until about midnight (76 degrees). Summer electric bill ~$180. Put the new handler (same outside heat pump unit) and last summer I could keep it at 75 degrees at night without an issue and the biggest bill last summer was about $120.
Those window units your are talking about are very inefficient.