I would vote for Amanithrax. My guild had only completed Blue and Green Basic tasks, and my hoard was at 134. I did have Zaejin at 14 power level. I ran Mushroom Man (died first room tho) and 3 King Gobtruffles. I suppose 4 Kings would have worked as well, especially if you luck out like I did and don’t have to face Legendary or Epic rooms on the way.
I also completed Silver Necropolis and Primal Rift without potions, but Amanithrax was the least stressful of the three. Primal Rift required getting Treant in 1st position, which can take awhile. Silver Necropolis required a lot of luck in keeping enemy Vanya from casting more than twice.