Flash is just inelegant on this card. It's like saying this is an artifact instant, only you can still cast it at the end of your turn without targeting anyways? Which seems a little above the curve?

The card is intended as a combat trick. Removing flash would completely change the card.

For a power level, it's one mana less than Moonglove Extract, which hits anything at any time, and is somewhat like Perilous Myr. I really don't think it's too powerful.

It obviously needs playtesting, but more for flavour than for strength, I think. There's not much to compare it to, but see, for instance, hornet sting (which is a baseline for "worst possible instant damage"), it's more flexible in that it can be played as an instant, or left on the board, but it's restrictive in what it hits. It seems about right to me.

You could compare it to Pit Trap, but this card has two fewer drawbacks than that for the same mana cost and a lower activation cost.