Not sure what to sacrifice? Here's a handy list...

2 Warlocks = 1 Goblin
2 Trolls = 1 Warlock
2 Goblins = Gold
2 Dark Elves = 1 Troll
2 Skeletons = 1 Dark Elf
2 Mistresses = Punishment
2 Salamanders = 1 Mistress
2 Bile Demons = 1 Rogue
2 Vampires = 1 Bile Demon
2 Rogues = 1 Salamander
2 Black Knights = 1 Vampire
2 Dark Angels = Gold
2 Imps = Cheap Imp Spell
2 Fireflies = Gold

Bile Demon, Dark Elf, Warlock = Create Imps
Monk, Monk, Monk = Mana Boost
Dwarf, Dwarf, Dark Mistress = Reinforce Walls