If you have fireball, the goblins you face won't be standard goblins. If you used fireball in a previous encounter, even monsters that had never seen that spell would know it was coming and would know that it's suicidal to form up in a handy AOE space.
If I hit something like this, the goblins would set some kind of trap, ambushes, be prepared to attack from all sides. Maybe have a few "brave volunteers" along with a bunch of straw dummies approach under the cover of a smoke screen to obscure the subterfuge. Or they set traps or just run away.