Any item that infinitely scales with one stack and no downside is too powerful. That's why Infusion got a cap of 100 HP per stack between RoR 1 and RoR 2. Why not give the Mystic Sword a cap like that, too?
"Killing a teleporter boss or an enemy with at least 1900 base health increases damage by 3% (+3% per stack), up to 24% (+24% per stack)."
If you insist on infinite scaling, I think the Metronome is fair because it's interactive and the bonus can be lost. Also, it doesn't scale past 100% crit chance without a specific red item. You could try something like this:
"Gain 15% more damage. Each time you hit an enemy with your Primary skill, you gain an additional 1% (+0.2% per stack) more damage. Missing an enemy resets this bonus."
or this, if you want to keep the giant slayer theme:
"Gain 15% more damage. Killing the Teleporter boss without being damaged by an enemy grants 15% more damage. Being damaged by an enemy during the Teleporter event resets this bonus."