Go to Root Earth -> Corrupted Harbor. Run towards the end but go towards the bottom area before the Blackened Citadel door. There will be a dead Root Walker with a blue mist around it. Use your staff on it to receive the Walker's Dream consumable. Hotbar this consumable to any of the 4 slots.
DO NOT USE IT IN YOUR CAMPAIGN. It sounds like you'll have to reroll your campaign if you've already used it.
Kill yourself with potion. Return to Ward 13. Reroll adventure mode (anywhere is fine). While still in Ward 13 on adventure mode now, use your consumable via the hotbar. It will return you to Root Earth to fight a corrupted Root Walker. If you walk too far in you'll spawn some other elites, so I stay by the entrance and kill him there.
Once dead you can pickup 7 corrupted lumenite crystals as well. Drink your death potion again to be returned to Ward 13.
Now you can repeat the cycle but speed things up a bit. Switch back to campaign mode but this time use your "Last Visited" checkpoint to return towards the end of Corrupted Harbor to skip the long run. There may be faster methods, but I wasn't happy with the bad RNG I was getting so this was more dependable for me.
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