Purity

Get down to 3 or fewer cards total across your hand, draw and discard piles at once. The clean way is a heavy Exhaust deck (Ironclad Corruption plus exhaust payoffs, or lots of one-off exhausting cards) - exhaust your deck down until only a couple of cards are left in play. A tiny deck helps, so this pairs beautifully with the small-deck challenge Minimalist - knock both out on the same run.