I'm always baffled when anyone relies on an auto dealership to do anything more complicated than exchanging a cashier's check for an automobile. If I had a nickel for all the blatantly erroneous information fed to me by car salesmen over the years I could use all my nickels to get myself a brand new Leaf.
The EPA range for your car, brand new, is 73 miles in ideal weather with minimal HVAC use. This is published all over the internet, on Nissan's website, on the car's original window sticker, on fueleconomy.gov, etc. So this is your upper ceiling on range, legally. No matter what the dealer said to you that's what the world's most perfect 2012 Leaf is capable of in perfect conditions, according to the EPA.
Your car is now missing a capacity bar meaning that it has lost at least 16% of its battery capacity. This implies that your upper ceiling on range today is about 61 miles (EPA rating, ideal weather, minimal HVAC).
If you want to "make the dealer hurt" or whatever, I'm assuming you mean to do so on legal grounds. Well, even in its degraded state, if a lawyer took your car out on a flat closed circuit tomorrow with properly inflated tires and set the cruise at a continuous 35 MPH with no HVAC and a with full charge in warm weather, it certainly would get 100+ miles of range. So can your car actually move itself 100 miles on a full charge? It sure can, with the right conditions. It's meaningless to you, but the car can propel itself 100 miles on a charge under absurd circumstances.
My opinion here is that unless you have something in writing from the dealer guaranteeing that your Leaf is somehow unique and will achieve 73, 84, or 100 miles of range with your driving style and patterns, that you have little legal recourse.
It's unfortunate, but I believe you put too much trust into the people with a vested interest in selling you a car while simultaneously doing too little to educate yourself about said car beforehand. Most of the people selling you the Leaf have probably never driven one more than ten miles. They may not have intentionally lied, they might just not understand their product and that's not a surprise given the blistering staff turnover at auto dealers. Nor do they care, as they'll forget about you about 90 seconds after you sign on the dotted line.
It's pretty common knowledge that early Leaf batteries degrade quickly, that the early Leaf heaters use a lot of power, and that the EPA published range (ideal, new) is 73 miles.
I'm of the opinion that the dealer is likely under little to no legal obligation to do anything for you, and if they do choose to try and improve your situation it's entirely due to goodwill on their part. You're at their mercy.
What you can do for yourself, given you got yourself into this predicament, is to make use of some tools widely used by the Leaf community to get the most use out of their cars. Get yourself Leafspy and learn how to use it. Make sure you have energy saving tires fitted and that they're filled at or above the rated pressure. Look for alternate routes that can reduce energy consumption. Etc. And the next time you go to buy a product in a segment you're unfamiliar with, know more about that product than anyone who works at the facility trying to sell it to you. It's a crummy system, and that's the only way I've found to protect myself.
2018 Tesla Model 3 Long Range (100% charge @ 43k miles shows 5.8% degradation)
2013 Nissan Leaf S + QC sold with warranty pack replacement (~35% degradation @ ~40k miles)
2015 Kia Soul EV+ Lease returned 10/14/17 45,000 miles w/ 13.8% degradation.