Lurking Deadeye

The condition is the whole trick: this Assassin only kills a creature already bloodied earlier in the turn, which turns a plain removal body into a cleanup mechanism for damage someone else has done. Flash is the ingredient that makes the clause reliable. Hold it up during your opponent's attack, let blockers trade or a burn spell chip a toughness, and the trigger destroys whatever survived carrying a point of damage. The 4/2 body reinforces the intent: it wants to ambush, not to sit on the table, since it folds to almost any retaliation once it has done its job. With nothing damaged, there is no legal target, so the ability never reaches the stack at all, and you have paid four mana for a fragile beater. That dependence on prior damage places it in a small class of removal that outsources its own setup: fight effects, first-strike blockers, a pinger, a stray burn spell, any source of a wound this creature then converts into a kill. It does not require combat, only a creature that has already taken damage, and the two most natural ways to supply that (a race on the ground or a point of direct damage) are exactly the situations in which you most want to be trading up. The card is priced as a reward for players already fighting, not as an answer you can point anywhere unprompted.