My oppenent killed my Testing Dummy with a Greater Amethyst Spellstone and every point of damage from the deathrattle hurt me a point, and was offset with a heal for my opponent. So I took 8 damage and my opponent took 0. Totally broken and cost me the game.
You confirm that the testing dummy was on your side of the board ? Your opponent didn’t steal it after you played it ?
Was this in Standard or Wild?
This spell has Lifesteal, so that might explain why your opponent was healed.