The main paths to XP are combat, healing, tagging zombies with a dna extractor, and reviving. There's no XP for barricading, which is why I told brittany that most survivor groups don't really expect people to spend time being useful until after they've gained a dozen levels or so. I mean, if newbies do try to be useful that's great, but until they reached the point where gaining XP just isn't all that important anymore then pretty much do whatever you can do to gain XP fastest. If you gotta kill zombies in the street, then even though that doesn't help anyone else, do that. If you gotta tag zombies, then even though that doesn't help anyone else, do that.
Of course, if you have the AP then the zombie spy in Whicher with me only has 30 HP...
btw may, I would suggest taking Diagnosis next. Although building your firearms skills is definitely important too, I would say taking Diagnosis next is even more important so that you can see the HP of everyone around you. That way you can see wounded survivors and heal them for XP. Pretty much everyone I've encountered for the past week or so has been wounded, so I'm thinking your notion that healing doesn't come up often is simply because you can't see who needs healing.
ADDITION: Now Darnell is literally the only other person in Whicher with me, the other survivors have skedaddled. The lights just came back on in Rowcliffe, we don't want to lose them again. He's only at 30 HP, so tempting to me, but I only have 12 AP (I'd want at least 20 AP to be sure of inflicting 30 damage using my pistols on an armored target.)
Even if nobody has the AP to kill him entirely, if anyone could even just hit him a few times to bring him down to like 20 HP and then post here I can finish him off.