Pick better blues and spell items. ANYTHING that does 4+ damage makes ghosts easy, and they're the only real threats that fight. The baron himself gives you 1 mana for every 2 damage, meaning even regular burst, 2 damage or 2 shield for 2 mana effectively costs 1, 1.5 mana against him. It's like most bosses, if you don't deal with the adds, you're dead. Have the high damage heros focus the ghosts first