After gaining access to the Battledome Challenge Endzone, use the Holoshield Launcher and Mini-Turret Launcher with Shock Mod equipped. After setting up the shields (to block the endzone from lucky enemies) and turrets, equip whatever other weapon you want to upgrade in the third quick-select slot, because as long as you have that weapon quick-selected, it will also gain experience without you even using it.

Complete the challenge and repeat it until you upgrade your weapons to the desired level, or get bolts to your heart's content. After being able to buy Experience/Jackpot Mods, max out the alphamods on the weapon of your choice to rapidly get them to lvl 99 and/or make millions of bolts.