Ring of Pain

When getting items from Mimic chests and corpses, there is an option at the top to spend souls to reroll the items you can pick from. You need to complete a run without rerolling ever (this includes free rerolls on the common items)

Best advice I would say is to pick up beneficial items as much as you can and be careful not to pick up anything that might mess up your build. A very useful item to get is the legendary item called Generosity. What this does is make mimics offer two pairs of items to choose from, meaning you can get up to two items from any chest, very useful when paying for legendary chests.
Try to build up your strength and go for optional dungeons to farm more items, the more good items you can stack, the better chance you will have to beat the final boss.