Destiny

This achievement is a long haul, It's taken me a long while to get all 3...and to be honest it can get pretty repetitive/boring as you progress.

All you gotta do is get experience, however there are many ways to this. In particular I did the whole game solo on hard for every mission. Although tricky, I was able to get bonus amounts of exp, whilst also honing my skills.

You have access to two 'Subclasses' the second of which you'll unlock at lvl 15 (though after doing this for your first character, the other two will be obtainable from the start!). Wait to do this one as you'll already have a lot of progress done for the basic subclass.

ALWAYS have bounties active. I do all story/co-op player ones available for the boost of exp, and it's very high.

Daily challenges are helpful as are weekly ones. Again you may need a friend or two to help you out.

Strikes are good for exp, but you might wanna get a friend or two to help speed things along. Me and a friend of mine did about 10+ strike playlists on the lvl 24 setting to speed thing along. We also got a lot of currency/nice loot.

You can also trade in stacks of 50x materials (i.e. Spinmetal, Helium, Spirit Blossom or Relic Shards) for a boost of exp, but will take you a while to amass the needed amount.

The cryptarch at the tower will be your port of call a lot, and as you bring him engrams he'll start levelling your cryptarch lvl. Doing this is easy (I'm as of writing half way through lvl 5) and nets you a big amount of exp.

Hope this helps guys!

Also; for those having trouble unlocking glitchy achievement do this:

Press the Menu button RB over to settings, highlight Log Out and choose Switch Profile (not Change Character). This will take you back to the game start screen, and once you press A, it will ask you to choose your Xbox Live profile.

Choose your profile and then choose your character from the character select screen. Any achievements that should have been unlocked will then unlock.

*Cheers to FLEEBS & ludo10109 for the info

The best way I found for getting this achievement, in addition to of course playing the game, is doing Nightfall each week as soon as it is ready. That gives you extra xp. After that, pick up all the bounties that you can, and do them. Vanguard ones were easier for me, but if Crucible is better for you, go ahead. After that, just work on weekly / daily. And once you have exhausted all the bounties you would like to do, and all the daily / weeklies are done, go ahead and do some Vanguard Tigers or play some Crucible. Within 3 days, you should have one subclass maxed.

The other guides cover this achievement very well. I just wanted to clarify something: I've seen many forum posts around the web claim that the Taken King subclasses (Sunbreaker in the case of Titans) do not count for the associated Mastery achievements. Today I unlocked this by fully upgrading Sunbreaker, so at the very least that is no longer true. This may be one of the achievements that was relaxed when Rise of Iron came out.