What’s the reward for completing missions

The basic rule for a single standing wide spreading portal: you start on top of the strongest resonator and once it is almost finished you move to the opposite side or if there is a high level resonator (say 6) which is not at the opposite side you should take it anyway.

You can do finishing blows from the middle, even from xmp 1 all resonators get 1-2% damage.

Actually portal takeover is very tricky and situation specific, sometimes you have multiple portals close by and this requires other tactics, sometimes you have unreachable resonators and this is something very different.

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.