Use the following trick to get the Bolt Multiplier to 20X and earn lots of bolts. To do this, you must have completed the game at least once so that you are in Challenge mode. Go to the Battle Arena on Planet Mukow and choose the "They Were On Sale" fight (with the hundred bomb bots). Once the battle has starts, use a Mr. Zurkow then start throwing the Nano Bugs (small turrets that shoot bugs). After that, use any weapon to kill what does not die from the turrets. The Pyrocitor is recommended if it is level 6 or above. Do this three times and your Bolt Multiplier will be at 20X and you should have plenty of bolts.

To get easy bolts and weapon upgrades in Challenge mode, go to the Imperial Fight Festival on Planet Mukow. When there, go to the area go to the place just before it and Swingshot to go to the fighting arena. Pick the weapon you want to upgrade and fight your way to the bridge to the arena. At the bridge, jump off. You will die and be brought back to the Swingshot area. If you do not get hit, you can get a 7x multiplier and about 2.5 million bolts on each trip.