There are two methods to doing this Trophy:
1. Purchasing Evie Frye from the Ubisoft Club Rewards for 10 Ubisoft Points
2. Look at the stats that pop up when investigating enemies with Ikaros from a camp/fort/ruin. Sneak up behind the Captain with Legendary stats and Press L3 in to Knock them out. Once knocked out Press L3 again to Recruit them. Once Recruited open the "Ship" Menu and go into special Lieutenants and assign your new Legendary Lieutenant and the Trophy will pop.