Shaman en-Kor

That free activated ability is the whole story, and it is a story about targeting rather than damage. The cost carries no mana and no per-turn limit, which means it can fire any number of times in a turn, each activation picking a fresh target among your creatures. The redirection it sets up is finite and harmless on its own (a replacement effect, not a damage source), so the part that breaks open is purely the act of choosing a target for free, over and over. Aim that redirect at a creature with a targeting trigger and the loop runs as long as you care to keep clicking: Cephalid Illusionist empties a library, Daru Spiritualist swells without bound, and this Cleric is just the zero-cost button that resolves the trigger again. No damage ever needs to be dealt; the ability only requires a legal target to resolve. The second mode runs the relationship backward, pulling a chosen source's damage onto this body to shield something you would rather not lose, but it lives in the shadow of the free-activation engine. The 1/2 frame barely registers. Unbounded zero-cost activated abilities are exactly the design modern templating has largely walked away from, and the en-Kor cycle survives as the canonical case study in how a free targeting cost, not the redirection it nominally provides, is the part that gets exploited.