This trophy is unlocked by taking one of the resources found in Cyrodiil near a keep.
To get to Cyrodiil and be prepared for this trophy if you are unsure on how to do this then follow my detailed instructions on the 'Alliance War Recruit' guide. Otherwise here's a short version:
- Once level 10 go to the menu and select a campaign in the campaign tab.
- Enter the campaign and find an enemy keep that is under attack by your alliance by looking at you map.
- travel to this keep by going to the nearest keep using a transistus shrine, then riding the remaining distance.
Once you reach the keep it is likely that you will take the keep before the resource because the horde of allies will be focused on that (at least in my experience). Remember that it is necessary to capture a keep for this achievement, but it does not have to be the same one as the one where you take the resource at. To do this, fight your way in with your allies to the central building and stand at the flag to capture.
Once you have taken the keep immediately ride to the nearest resource (it will show up on your compass) and attack it (don't attack it alone, or even with two people. Wait for your allies to show up.) Once you have defeated all of the enemies at the resource it will be taken and the trophy will pop.
If you have a group of 5+ good players then you may attack without you main alliance horde, but otherwise don't. If you die then you will have a long walk and it may be taken before you get back (you can't revive on the spot unless an ally does it for you).
Note: (unconfirmed) it may be possible that if you die then you can lie there dead while the resource is taken and still count it as taking it (as long as you participated in the battle). This is based on the fact that I once died during a tough enemy fight in a quest, and another guy finished him off while I was dead. I lay there without respawning and the quest was progressed. If someone can test this and confirm/deny it then that would be useful.