Basic:Complete public events around any of the planets 10 times with a gold tier rating to unlock.
Extra Information:What are public events?Public events are "random" events that occur when
patrolling.
It's also possible to run into these during the open world parts between mission instances too since you are essentially put in the shared world where they occur. They ask you to do things such as
take down the devil walker/Defend the Warsat/Kill X before it gets to X /Defeat the extraction crews and Prevent the vex sacrifices etc etc. They generally will have some form of time limit to accomplish them.
You will know one has occurred as the sky gets very dark momentarily and a notification/way point should/will appear directing you to it.
The highest tier you can get for completing one of these events is
"gold". This can be done alone (Especially higher levels on lower level worlds like Earth etc) but of course it is much easier if others join in. Be it fireteam or randoms in the world.
So far it seems completing the event before the time runs out should net you the gold or the majority of events.
HOWEVER, some do appear to need something more. For example the event to
stop the vex sacrifices on Venus. Simply allowing even just 2-3 vex sacrifice themself will net you only silver-bronze so you need an almost perfect run at it for the gold. Thanks to
HolyHa1fDead for that info.
If you manage to
complete these events with a Gold tier rating 10 times it should unlock.
Spawn times:EDIT:I got a few messages about this link: It's a handy site that tells you (with countdowns) which event is due when and is auto updated so you need just check/watch the site
http://destinypublicevents.com/. It's not infallible but it often provides good rough times.
Using it as a guide it may be possible to make a circuit and grind out
vanguard marks. You receive
4 marks per gold tier for example. (Three for silver and 2 for bronze.)
It should also be noted that sometimes instead of a PE you'll get the message "The enemy is moving against one another" and an AI enemy fight will break out instead.
Also
DesertFoxHunter has a solution for a one area grind if you want to work through bounties, do public events and get the achievement. You can find that under the following spoiler also.
*** Spoiler - click to reveal ***
The easiest I found to grind so far was to start off on Earth, go to explore cosmodrome, and as soon as you spawn, head to the right.
You will run through a building and come out on the other end (the area before you get to Devil's Lair/the area you run through at the very beginning of the game.
I hung out on the roof of the building in the middle of the map (the one by the sign that has the dead ghost, which you can also get if you weren't aware) and then just hang back and kill weak enemies and work on kill bounties or precision shot bounties until the event spawns.
The event is always the same, it comes every 20-30 minutes (rough estimate, don't really time it, sorry) and it's always a devil walker that drops in the open field. If you sit on top of that roof and are high enough level this should be a cake walk for you to complete in 5 minutes. Once you complete it, hang back and wait and eventually in about 20 minutes another one drops. Rinse, repeat. Time consuming, but easy and effective for me. Again, it'll always be the devil walker in that area, and if you're level 12 or higher it will be a breeze.
Added timed confirmation from them:
Entered map - 23:40
Incoming public event - 00:16
Completed - 00:18
Incoming public event - 00:45
Completed - 00:48
Incoming public event - 01:13
Completed - 01:16
Of course you can also farm one area like this at the loot cave should you be so inclined. (Note the cave has now been patched to only spawn every 40 seconds).
It was mentioned that it wasn't working for at least one person but
Exalted Entity has mentioned in the comments that sitting idly may not get it to spawn and that someone must be killing the enemies around the area in order for it to trigger ("Calling for backup" as you will). Thanks!
Achievement popping problems:It should also be noted that people seem to be having trouble unlocking some achievements on this game. You should try going to settings, log out, switch profile, re select your profile and they usually pop just after selecting your character.
You can also try exiting the game fully (not just dashboarding but pressing start and exiting the game whilst on the dash) and starting it up fresh or even a hard reset of your xbox too if the others don't work.
Extra info: I added a video found on YT by
Leon Kaspers. It shows you the sky getting dark prior and shows the fireteam getting a gold taking down the devil walker public event. Just if anyone wanted visuals/examples.