I did this way to grind for better equipment since the game have low info about how to do it.
First of all, upgrade the double drop perk to the max asap then move to the second map. There you gotta find a bonus room with boxes of cores. This room often shows up.


This is the bonus room. Make your way to the top.


Up there you gonna find these cores. If you have the double drop maxed can often drop them doubled.
grab them and don't think twice on fusing them on a better gear. Level them up to unlock them fast in the safehouse and then keep recycling for better gears.

There's also others rooms that drops golden items with 100% rate.



Solving these rooms puzzles will grant a golden item

Now the profit part. Pick the golden ones, better level them up first. Found that the golden gears are faster to level. Pick two of them. Don't need to be of the same, and fuse them on a better item. Since those golden drops are always there and always the same, you can spend them without worrying about having to pick them again. This way you can quick farm better items for harder fights or that advanced craft recipe.

Easy, huh?!
I hope that could be useful. I wish i knew that when starting the game. Would have saved me a lot of trouble xD
Enjoy.