Lifegain decks are built around triggering lifegain with cards like Case of the Uneaten Feast, Haliya, and Enlightened Confidant, so Sunspine Lynx's 'players can't gain life' ability shuts off those payoffs, including the Amalia Benavides Aguirre and Moseo lifegain triggers. Its ETB deals damage to each player equal to the number of nonbasic lands that player controls. That hits their Godless Shrines, Shattered Sanctums, and Concealed Courtyards, but our own nonbasic-heavy mana base takes damage too, so count both sides before deploying it. Duress can only take a noncreature, nonland card, so it can't touch their creature-based engine pieces like Liliana the Faultless. It is good for stripping Erode before it can answer our threats, and for taking Ajani Resolute, Lunar Convocation, or Case of the Uneaten Feast. Stinging Vitriol's damage and discard are weak compared to disrupting their engine directly, and Valley Flamecaller's bonus damage matters less when Sunspine Lynx stops their lifegain from keeping pace. Deploy Sunspine Lynx as early as possible to cut off their Amalia Benavides Aguirre and Moseo engines, keeping in mind that Erode can remove it.