Compared to Batman, Green Arrow doesn't have anywhere as many enemies. However, one of Oliver Queen's most enduring antagonists has been the dark archer Merlyn. A member of the League of Assassins and highly skilled marksman and mercenary in his own right, Merlyn has crossed paths -- and swords -- with Green Arrow on multiple occasions. Their shared antagonism has escalated over the years. However, when offered a demonic chance at having his deepest, darkest wish granted in exchange for his soul, Merlyn chooses not to take the opportunity to best Green Arrow once and for all -- instead, he wishes to challenge the Dark Knight.
Created by Mike Friedrich, Neal Adams and Dick Dillin in 1971's Justice League of America #94, Arthur King takes on the stage name Merlyn the Magician and wows the world with his archery skills, including a young Oliver Queen. This inspiration backfires on Oliver when Merlyn challenges Green Arrow to an archery contest years later and publicly bests him. Returning as part of the League of Assassins, Merlyn attempts to take out a contract on Batman's life, only to be thwarted by the Green Arrow. Oliver closes the skills gap between himself and his former inspiration, thereby kickstarting the two's longstanding rivalry.
During the crossover event Underworld Unleashed, the demon Neron appears to the heroes and villains of the DC Universe with an offer to grant their greatest wishes in exchange for their mortal souls. Following their agreement, in which Merlyn wishes to kill Batman, he joins a group of assassins known as the Killer Elite -- including Deadshot, Monocle, Deadline, Bolt and Chiller -- to carry out various assassinations across the DCU. This leads to Merlyn's obsession with murdering Bruce Wayne.
The plan is thwarted by the entirety of the Justice League, though the villainous ensemble continues for a time, even after Neron's defeat by Shazam at the conclusion of Underworld Unleashed. The team of contract killers -- along with Merlyn's ambitions of killing the Caped Crusader -- eventually disband after their defeat at the hands of a different, short-lived villain team known as the Body Doubles, who are a pair of rival assassins working for the nefarious Requiem, Inc.
Another major reason for Merlyn selecting Batman as his dream target when he accepts Neron's offer is due to significant changes to Green Arrow at the time: Oliver Queen had just been killed. In 1995's Green Arrow #101, published one month before the start of Underworld Unleashed, Oliver heroically sacrificed his life to stop a terrorist bomb plot. While Oliver's son Connor Hawke eventually takes up the Green Arrow mantle, he hasn't built up an antagonistic relationship with Merlyn when Neron makes his offer.
With his greatest enemy dead, Merlyn chooses to pursue the target that has eluded him since he joined the League of Assassins. After all, you always remember the one that got away.