To get this you must complete the secondary objective of the mission. This isn't fail condition so if you do lose it then you can still continue the mission.
However I dont really see how you could lose it... I mean I was on standard difficulty so I recommend that as well.
There are some cannon galleon in the beginning that could give you trouble but you start with a decent navy to deal with them. Other than that, purple attacked me once with 3 battering rams but that that was easily prevented as well as the towers to defend the wonder do a lot of help.
Admiral Yi can be reached in 3 ways
-Sail around from the west corner of the map
-Capture a transport ship to the north between the admiral and purple on the edge of the map
-Use an Onager to clear the forest around the gate (not Mangonel)
He unlocks an army that you can transport off an island and kill purple with
He also unlocks Tank like ships that you can then also create from your docks.
So to quickly do this scenario:
-Get a castle for defense and spy and HP upgrades
-Do upgrades from university to unlock bombards and upgrade the HP of buildings
-Fortify your docks with a lot of towers (10-12)
-Get Admiral Yi in the mean time
-Get a good navy of fire ships, Turtle Ships and Cannon Galleons and raid yellow docks
-Kill purple with the island crew