First off, do you have a case on your phone? If you do I would check to make sure that it is allowing the phone to breathe properly and isn't trapping a bunch of heat. This is pretty easy to test by just taking off the case for a little bit and seeing if it runs cooler without it.
Another solution to your problem would be to limit how many applications you are running at one time. Try quitting an application when you are not using it. Other known reasons are that watching videos may cause the phone to heat and the phone usually heats up while it is charging. Try lowering your brightness and turning off settings such as bluetooth and background app refreshing.
If closing applications and checking your case does not work there is a slim chance you may need to replace your battery entirely. Try replacing the battery with a new one and see if the charge lasts longer. If not it may just be your phone. The LG Spectrum does not have a good reputation for overheating among other things, therefore it may not be your fault that the battery doesn't last long.