You started the tit for tat by assuming the players were idiots. I'm just saying that if it's the goal of the GM to produce an interesting and challenging encounter with 18 orcs versus a 6th level party it's trivial to do so.So you are backing off your claim that 18!orcs being hit by a fireball makes someone a bad dm?
For those suggesting orcs are too stupid to avoid clumping up: orcs are murder machines that live in a world full of people and things that can kill you in groups. They probably have one in their ranks or work for one or have to deal with ones that hunt their territory. Spell casters and creatures with breath weapons and all sorts of horror. Even a troupe of baboons spreads out and flanks prey and they don't have to deal with fireballs.