You've finally found a handful of keywords that you know will get you the kind of traffic and conversions your site needs. There's only one problem - your client/boss disagrees. While you're focusing on actionable keywords, they're stuck in branding mode. Or as I like to call it: professional blindness. You see it's a problem we all run into over and over again. The problem is, many of us don't know the best way to go about proving our point. After all, it's hard to not give your client/boss what they want. They are signing your paycheck, after all.
So what's a guy/gal to do? The secret is: testing. Ideally before you even present the recommendations to them. You see, it all goes back to that rule we all learned the hard way: it's easier to ask for forgiveness than permission. The same rule applies to SEO. Your boss/client has their mind set on exactly what word they want to show up for in the search results. It's something their boss wants and it's something they think about every day. It's your job to show them that while it would be great to rank for [insert broad keyword that has nothing to do with their business], it's just not practical. Especially since the traffic most likely won't be converting. That's where testing comes into play.
SEO is a walking race. It takes time to rank well for a targeted keyword (most of the time, anyway). PPC, on the other hand, gives you the freedom to start collecting data right away. That's the secret to getting what you want. Start a mini campaign for both the keyword you want to target, as well as the keyword your boss/client want you to target. Make sure you are sending them to the same landing page you would be targeting with your SEO and that you have your conversion tracking in place. The next step is simple: collect data. Get at least a month's worth of data so you'll have enough to make your point.
Now it's time to analyze. Take a look at what the traffic is doing on your site. Look for more than just conversions. Are your visitors engaging with your content? Were they looking around at other pages on your site? There are lots of questions you'll want to ask so you'll be prepared to use them as part of your case.
Once you've analyzed the data it's time to share the results. Your goal is to make a quick one page report that include lots of pretty graphs that show the performance of the keyword you want to target and the one your boss/client does. Include as many financial figures as you can. If you can say to them, "X keyword will drive us an extra $1,000/mo in revenue, while the keyword you are interested in will send us $10" - you'll find yourself leaving with a victory.
It's a lot of work, but it's necessary. It's hard for clients/bosses to see the big picture. They have a certain conviction with the keyword they selected. It's their baby and they'll do anything to defend it. That's why you need to put together a straightforward case to make them realize how much money they're leaving on the table. If you go to them before testing this out you may lose out on your opportunity to do the test in the first place.
Above all, don't back down without putting up a fight. They may still decide that they'd rather lose that extra $990/mo so they sleep better at night, but at least you helped them make the most informed decision they could.