Chain deals no damage itself — it is a pure setup card. The prime window is when you already hold Fire Attack or Lightning: link two enemy seats, then unleash the element so it lands on both. Rebels ganging up on the Lord love it, as does anyone wanting to clear multiple targets in one turn. You can also unchain yourself defensively when a burst is coming down your row.
Its true partners are Fire Attack and Lightning — chain two targets, then set off the element to strike both. If you run Huang Yueying, playing Chain (a trick) also refunds you a card through her wit, keeping the combo engine fed for the follow-up burn.
Negate cancels the Chain outright, but it is usually smarter to save Negate for the Fire Attack or Lightning that follows, since Chain itself deals no damage. Once linked, avoid sitting at low health next to another low-health seat, and keep a Peach in reserve to heal after the elemental burst lands.