The idea is interesting, there are three main candidates for the card, all of which might be a bit too powerful in terms of CC. Eclipse Parasite Upped, Flying Lobotomizer Either, and Flying Owls Eye Either.
All of these have at least 5 Atk, with the pact being no cost flying owls eye is a mono unupped.
With the formula for killing as it is at Atk+1 / Hp of creature, and that it adds +1 or +2 Atk, we come up with instantly killing any creature of up to 7 Hp in Unupped, and 8 in Upped. And even up to 12 or so HP, which is the Max Hp of the highest non-gravity creature in the game, it kills about 2/3rds of the time.
The primary downside is that this always requires at least a 3 card combo to pull off optimally, and that it is extremely fragile (In upped killing the eclipse actually kills otherwise unbuffed parasites, and the others involve flown weapons which reverse time nukes to hell and back). I'm not sure on my judgment one way or the other, just felt like throwing the numbers out there for kill chances.
On a side note, this does allow for Mono-Time neurotoxin decks Upped.