All you have to is earn 45 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 (Veteran difficulty):
Campaign Levels:Southport:The Outskirts:Pagras:Twin Rivers:Silveroak Forest:The Citadel:Goldstep Mines:Icewind Pass:Stormcloud Temple:The Wastes:Forsaken Valley:The Dark Tower:Ruins Of Acaroth:Hushwood:Bandit's Lair:Sarelgaz's Lair:Glacial Heights:Ha'Kraj Plateau:Rotwick:Ancien Necropolis:Nightfang Swale:Castle Blackburn:Rotten Forest:Fungal Forest:Pit Of Fire:Pandaemonium:Heroic Challenges:Southport:The Outskirts:Pagras:Twin Rivers:Silveroak Forest:The Citadel:Goldstep Mines:Icewind Pass:Stormcloud Temple:The Wastes:Forsaken Valley:The Dark Tower:Ruins Of Acaroth:Hushwood:Bandit's Lair:Sarelgaz's Lair:Glacial Heights:Ha'Kraj Plateau:Rotwick:Ancien Necropolis:Nightfang Swale:Castle Blackburn:Rotten Forest:Fungal Forest:Pit Of Fire:Pandaemonium:Iron Challenges:Southport:The Outskirts:Pagras:Twin Rivers:Silveroak Forest:The Citadel:Goldstep Mines:Icewind Pass:Stormcloud Temple:The Wastes:Forsaken Valley:The Dark Tower:Ruins Of Acaroth:Hushwood:Bandit's Lair:Sarelgaz's Lair:Glacial Heights:Ha'Kraj Plateau:Rotwick:Ancien Necropolis:Nightfang Swale:Castle Blackburn:Rotten Forest:Fungal Forest:Pit Of Fire:Pandaemonium: