So the following is a bit of cheese, but it checks out ruleswise. Just don't be surprised if your GM hates it. Also it's got a pretty low success rate. But if you're lucky, this could be great.
Step 1) Learn Telekinesis.
Step 2) Buy as many Dragon Slaying Arrows as you can afford. Greater Dragon Slaying Arrows = better.
Step 3) When fighting the dragon, cast Telekinesis and use the violent thrust option to launch as many as 8 [greater] dragon slaying arrows at the beast.
Step 4) Revel in your wizardly might as the dragon dies a horrible death.
Pros: This is a lot of damage in one round. Assuming all your attacks hit, the dragon takes 8d8 damage. It will then take 50 damage for each failed Fort save. That's up to 8d8+400 for slaying arrows or 8d8+800 for greater slaying arrows.
Cons: Dragon AC's tend to be high. So don't expect all your arrows to hit. Also, dragon Fort saves also tend to be high. So expect it to fail against many of the arrows, if any of them. Lastly, slaying arrows are EXPENSIVE for one use items. But since this is end of your campaign, might as well spend everything you got.
This is tactic is probably best used as an opening salvo or a last resort. You definitely don't want your entire group's plan to hinge on this one crack-brained idea.