I don't get why this card should have so weak effect with so high cost. Discarding a common if Burning tokens set and ability to trigger Illusion keyword aren't so great. Compare it with Mermaid. 5R, -2 enemy tower, -2 enemy wall, -2 enemy stock and potentially +5 wall, +5 tower and additional -3 enemy stock.
I think Wisp should cost 3G.