You can do this with Gnaw on the map Ribbit IV in a custom game. Ensure the "Game Mode" is set to Awesomenauts before starting and DO NOT touch any settings. (Otherwise it disables achievements) You can change the difficulty of the bots, but nothing else. So the solar bosses are in the middle of the map between the two lanes. One at each central piece of the map. They spawn at 2 mins so make sure you're there to greet them when they do as you don't have long for the kill. Here is a video to help:
Credit goes to HCmacko for the video