In Ingress, when are links destroyed

Hack plenty of enemy portals. You get 100 AP for hacking an enemy portal, but none for hacking a friendly one.

If there are no unclaimed portals around (highly likely in some locations), find a level 1 enemy portal and start slugging away. You'll want a few dozen XMP bursters at minimum to destroy the portal, but the rewards are high--destroying 8 resonators and planting 8 of your own on a single portal will get you almost a quarter of the way there. Even if you don't manage to capture the whole portal, you'll still have gotten 75 AP for each resonator you destroy.

Deploying portal shields is a good way to get AP that may not be obvious at first. Check every friendly portal you encounter to see if it has a full complement of shields. If it doesn't, deploy as many as you can. You get 150 AP for each one.

Turned out that the inability to deploy the 8th resonator was a bug - I wish I'd taken a screenshot. When I got back to my computer, I looked and 8 had been deployed. It must have been I kept tapping deploy on my phone faster than something could handle it, and it just didn't show up. I'm wondering if restarting the ingress app on my phone would have fixed the issue as well.

As far as being unable to link it could have been for a number or reasons that I didn't know at the time. Portals link distance is determined by the level of it's resonators, and I may not have had sufficient resonators.

Level   Range
1       160m
2       2.560km
3       12.96km
4       40.96km
5       100km
6       207.36km
7       384.16km
8       655.36km

It also could have been because the link I was trying to create crossed the link between some other portals. It wasn't obvious to me that this wasn't possible at first, but makes sense and you get a message telling you why you can't link to a portal if you click confirm even though it's greyed out.

Finally, it could have been because portals only allow a certain number of links. You can only have 8 outgoing links and 8 incoming links. When this happens you'll see something like "Origin Link Capacity Reached" when you try to confirm a link to an unlinkable portal.