So I was playing challenge mode yesterday, after I finished the game and discovered that when fighting Blargian Snagglebeast, all weapons level up much faster then using them normally, on smaller enemies. Then killing myself/reloading checkpoint, rinse and repeat. Also make sure, to change the difficulty to hard and there you go. However Sheepinator takes forever to level up, right after that section on planet Gaspar, I think a little while in the level you must destroy the gates with turret gun, once you do, 3 big robots come out from the door and immediately use Sheepinator on them, levels up slightly faster than using normally, simply kill yourself and repeat the process.
Also following glitch to get 10 million bolts, lots of cards and almost 1000 raritarium crystal, still works. Will make the game less grindy + also "Cha-Ching" trophy will pop.
Here's the link to the video: