You cannot reduce level 100 item to level 20 but it used to be a feature back in 2.1 and before. It didn't seem to be intended but basically you could go to floor 20 and level down your items to level 20. It was removed for many reasons and because peridot isn't supposed to level down.
But you can get level 20 items for EXP farming and still add cruicial affixes. You just need a legend/loot with level 40 that are found at floor 20 or so for the level 20 character to wear .
Then add 6 Zircon for sockets and add 6 quest stones. Then add affixes like Ignore resist , Plagued, Poison, fast moving skill, move speed and some amount of damage.
Using that poison build for a packsize at that level on floor 100 provided you survive will get you tons of levels fast enough to reach level 50 from level 20 gear on solo mode .