is this a good battery??
 

had to purchase a new lipo for my losi mini lst 2 with a brushless castle creations 6800 kv, the hobby shop i went to didn't have any and told me they are popular in airplanes so i went and talked to a guy at an airplane specific hobby shop. he gave me a Pro Power 30c 850 mAh 3s 11.1v. just wondering if this is the best battery i should be using and how much run time i get out of it?

an 850 mah 3s with a 6800 will be ballistic fast, will probably run hot, and should get you about 6-8 minutes runtime depending how hard you're driving

should i invest in a different one?

That's up to you, and what you'll be using it for. There are plenty of 1300-1800 Mah lipos out for 1/18 now. If it was mine I'd look for a higher capacity 2S.