At the "Continue" screen, select the "Password" option. Enter "JUSTIN BAILEY ------ ------" as a password to resume the game partially completed on the Hard difficulty with all upgrades and maximum money.
Go to a map with a shop that is near the entrance. Buy upgrades for any or all of your three summons. Then, press Start, and select "Restart Level". You will still have all your gold and your summons will have the upgrades that were purchased.