All you have to is earn 30 stars, you can earn up to 3 stars in campaign levels, 1 star in heroic challenges and 1 star in iron challenges. The videos below might help you out (Impossible difficulty):
Campaign Levels:Gray Ravens:The High Cross:Waterfalls Trail:Redwood Stand:Royal Gardens:Gryphon Point:Rockhenge:Grimmsburg:The Crystal Lake:Neverwonder:The Unseelie Court:The Ascent:Arcane Quarters:Mactans' Retreat:Shrine of Elynie:Galadrian's Wall:Blood Quarry:Beheader's Seat:Dwaraman Gates:Tainted Pit:Duskwood Outpost:Duredhel Outskirts:Heroic Challenges:Gray Ravens:The High Cross:Waterfalls Trail:Redwood Stand:Royal Gardens:Gryphon Point:Rockhenge:Grimmsburg:The Crystal Lake:Neverwonder:The Unseelie Court:The Ascent:Arcane Quarters:Mactans' Retreat:Shrine of Elynie:Galadrian's Wall:Blood Quarry:Beheader's Seat:Dwaraman Gates:Tainted Pit:Duskwood Outpost:Duredhel Outskirts:Iron Challenges:Gray Ravens:The High Cross:Waterfalls Trail:Redwood Stand:Royal Gardens:Gryphon Point:Rockhenge:Grimmsburg:The Crystal Lake:Neverwonder:The Unseelie Court:The Ascent:Arcane Quarters:Mactans' Retreat:Shrine of Elynie:Galadrian's Wall:Blood Quarry:Beheader's Seat:Dwaraman Gates:Tainted Pit:Duskwood Outpost:Duredhel Outskirts: