Use Goblin Gang on defense against single-target cards like Mini P.E.K.K.A, Prince or Hog Rider. On offense, sending it at the tower while your opponent's small spell is out of cycle deals real damage. With Goblin Barrel it overloads your opponent's spells.
Tips
Use it as bait to pull out your opponent's small spell.
Spread it around a single-target tank to melt it quickly.