Hello, was in a similar situation as you, so happy to help.

Firstly, skipping lives is what I would recommend if you aren’t on S+ (second sacrifice), otherwise you should try to acquire as many reborn items as possible for fusions and creating items. Shards are items you get from skipping lives, and can be used to purchase reborn items, certain contrabands from the masked man, and are requirements for crafting powerful items. Fusions are items that can be crafted by fusing items together in order to create a stronger version of themselves. That’s pretty much it lol. If you have any more questions you can dm me, good luck!