You can get 4d6 apiece easy from colossal sianghams or crossbow bolts. If you're willing to shell out actual money (as opposed to the minor cost for nonmasterwork weapons) buy a Metamagic Rod of Chain Spell, you can do some really serious damage.
FTFY. Also, just use colossal arrows. You can hit 50 of them at once with a Greater Magic Weapon + Flame Arrow. No MM Rod of Chaining needed.
Edit: Teh maths.
colossal arrows do 4d6
+5 each from GMW
+1d6 fire each from Flame Arrow
Avg per arrow = 22.5
*15 arrows = 337.5
So yeah, not really broken.
You could instead use Minor Creation (Black Lotus Extract), and then just chuck flasks full of
that with Telekinesis. Or alchemist's fire (is there a way to make this temporarily for free?), or Fire Seeds ("throw" all those Holly Berry Bombs at once
). That gets around having to hit their full AC, instead of touch AC.
Or use other spells to add "save or die/suck" effects to each arrow (or flask...).