Wilkinson had never met Legolas. Yet, as the student of the Pharmacist Guild's President, Balbo, he frequently heard conversations between his mentor and other Legendary powerhouses. Hence, he naturally had knowledge of many random things. One of them was regarding the Legendary elf mage.
Legolas first entered the human world many centuries ago. He was a diplomat for the newly established Elven Kingdom who wanted to negotiate matters regarding an armistice as well as the issue of hostages.
All the kingdoms suffered significantly in that prolonged war. Hence, when they heard that Legolas from the Elven Kingdom was there for negotiation, they wanted to establish their dominance. How could an elf be any threat to them without his forest?