How to win with ease. (No Artifacts.)
One Machine Gun and one EMP, the rest being Shields.
Place the EMP and Machine Gun on the part of the world that is starting to come into view as round starts, make sure the EMP comes out first so you can kill Shields, Black Holes and Railguns.
This will let you have the maximum amount of time to empty rounds into the planet.
Level up Machine Gun shots fired per volley and reload speed up to three levels then damage up to three levels. Next max reload speed then balance damage and shots fired per volley upgrades until Machine Gun is maxed out. Now upgrade the duration of the EMP and reload speed to level three. Once that is done, upgrade the reload speed on the Shields to level one or two so you can block a little bit more. With a little luck, you should now be able to kill an enemy planet before your Machine Gun even leaves the screen.
Feel free to upgrade whatever you want with the rest of the money.