I'm thinking about giving it an internal limit of 3-4 triggers per stage, and a guaranteed trigger on TP bosses.
Viewing post in The mystic sword item is very powerful!
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."
Yeah, the more I'm thinking about it, the more I agree that it should have some sort of cap. I'll probably make it something ridiculously high like 100% max damage boost per stack, so it's high enough to not really affect the player in early game, but low enough to not give the player 10k% damage from one item.