Reminds me of the Morph-Mechanic. I don't know to what extend it is really necessary to deviate from morph, but it works just fine I think...
But you do realize, that you can kill off any targeted creature, not just the one blocked by your card?
The only actual corrections you should take are with the color alignment. "Destroy target creature" is something you almost never have on a green card. On the contrary, "Destroy target non-creature permanent" is more or less a trademark line of green. As it is now, your card feels black, but you could try to either just give it Deathtouch (allowing it to be either black, green, or a combination), or instead of its current flip trigger change it to
"When a creature your opponent controls attacks, You may turn [THIS] face up. If you do, it fights that creature.", which would make your card as green as it gets.