The fastest way I've found thus far of killing the prismatic dragon purely with attacks involved the arcane duelist's flurry of blades. That thing seems to be seriously overpowered, even if it is limited to one use on the dragon per day. I suppose if you ran a Hexblade/Arcane duelist/Weapon master with a high crit weapon like a rapier, you might do fairly well: You'd have improved critical, ki critical and keen acting on a 18-20 threat to be begin with, so your 1d4+3 images should have no problems hitting it multiple times in one round. It's just a pity you can't toss in sneak attacks as well. You'd also get dextrous attack to play with, but I don't know how good that actually is, or even whether it'd stack with a +20 weapon.
If you'd rather not play with images, I suppose you could look at the strength bonus route. The mighty contender of kord seems like a good choice for that, but I've never actually tried one.
Also, the Shining blade's class ability (brilliant blade, I think) is another thing that provides +20 attack and adds some other damage types to your weapon, but it only works with a longsword.
