There is no diet that can help you spot reduce weight...But you should probably eat healthier and exercise anyways.
No, paleo won't help you lose fat specifically in your lower chest region.
Properly implemented, paleo can help you lose body fat overall. With overall fat loss, hopefully, will come the loss of your lower chest fat. Were you heavier overall all & the chest fat is the last to go? Or are you still uniformly fat all over?
If you want more specific suggestions you need to provide more details.
It's all about the details.... weight, height, waist, body fat % (estimate, guess, measurement?) How heavy have you been? How much fat have you lost? Over what time period?
Currently eating...foods & qtys
Exercise... intensity, duration, frequency
Any extenuating circumstances or health conditions..
by 'decent' shape, I assume you mean good strength / musculature but potentially still hidden by fat?
Google body fat images and click on Built Lean images... it'll give oyu an idea where you stand. If you're north of 25% you've got 'easy fat' to lose. The lower your number, the harder to lose it. :(
Also take a look at waist to height ratio... imo way better than BMI about giving an indication of 'fitness'. If your waist ~ a top of pelvis / belly button height is more than 50% of your height....you've got fat to lose. If you're really in decent shape (good muscle & low body fat) your waist should be more like 46% of your height.
In order to burn chest fat you need to follow strict diet. Try to consume lean meat, fruits, and vegetables with good amount of protein obtained from protein shakes. Start focusing on cardio treadmills, cross trainers, rowing machines, diamond push ups etc are good for burning lower chest fat.
Sounds like you've been exposed to too many xenoestrogens such as soy and now you've got moobs. :( Not much you could do other than get as lean as you can, and if it still doesn't go away, consider surgery.
Since that dude doesn't really understand the struggle of hating your body, I'll help you out bro. A very low carb diet is what will help you lose your excess body fat. Following a paleo lifestyle will help even more. However, you must educate yourself on how everything works. I'd recommend getting a few of the top paleo books and reading them. It's hard at first, but I've come a long way since I was 18. You can do it.