Dragon Age: Origins

If you start your game with an imported character from the main campaign, you'll be at the same level (somewhere between 20 and 25 for most people with a "post Archdemon" save).
With a new character, you'll start at lvl 18.

I've been playing with an imported lvl 22 rogue and got my lvl 30 around the middle of Awakening.
Right now I'm on my way to the final boss and just a bit over lvl 34.

So don't worry too much about that achievement, it'll come eventually.
Just make sure to complete as many side quests as you can, unlock safes/doors...

edit : you have to hit lvl30 while playing Awakening or it won't unlock ! (thanks for telling me guys).

Whilst the other guides are good & have their merits I came across a glitch method that nets you 2000XP every 2-3 mins & involves no killing,1 sentence of dialogue & a quick run through the 'Keep's basement.

One of the 1st quests in DAO:A called 'It comes from beneath' requires you enter the 'Vigil Keep's' basement,slay a few enemies until you reach a caved in tunnel-this is where the glitch starts.Sergeant Maverlies will appear & after a brief conversation will ask if you're ready to go back to the keep.Agree to this & you'll return to the entry door to the basement.Simply re-enter the door behind you,go through the now deserted basement down to the caved-in tunnel & Sgt Maverlies is there again,approach & talk to her & she'll ask if you're ready again,accept & another 2000XP-just rinse & repeat.

Credit:Eskills x360a.org

EDIT:Just to clarify,Maverlies will return above ground with you-DON'T SPEAK TO HER ABOVE GROUND-Just enter the door,go down to the cave-in tunnel where SHE ALSO APPEARS & talk to her here-this is why the glitch works,her character remains in the caved in tunnel until you speak to her ABOVE GROUND.

Be warned that by completing this particular line of quests you will be unable to complete any further quests for the city guard in the city of Amaranthine. You must have the ‘Awakening’ DLC installed which can be accessed at any point from the main menu. When you first arrive in the city of Amaranthine and before entering the main gate you will notice a ‘shady character’ running around frantically. He will ask you to go speak to the bartender at the tavern ‘The crown and lion’ and persuade him to clear a passage which is currently blocked. You must be strong in Coercion & cunning to persuade or intimidate the bartender effectively into clearing the passage. Once you have done this you must return to the ‘shady character’ and choose the conversation which says “Anything else you need from me?” He will then inform you that a group of thieves have been having a negative effect on the profits of the smugglers and explains that they have attempted to run them off but have been unsuccessful in doing so. Eventually he will ask if you could help find a more permanent solution to which you should choose the option that says “you mean kill them." Next he tells you that he has arranged for his associate to meet with you inside the den of thieves to help you clear them out; you must then go to the den which is nearby and clear out them out with relative ease. Once this is done it will then become possible to take advantage of the glitch which will grant you exactly 15 sovereigns every 5 seconds or so. You will also gain a small amount of experience for each member of your party.

Speak with the smuggler leader and he will reward you with the 15 sovereigns; choose option 3 (is there anything else I can do). He will then ask you to kill the lieutenant in charge of the city guard to which you must respond with option 3 again (I can’t do this). You can now continuously do this over and over until you become insanely rich along with leveling up fairly quickly at the same time. I managed to generate around 100 sovereigns every 30 seconds as well as going up 2 tenths of a level at the same time. You could just never accept his request to kill the lieutenant and return to him whenever you need an injection of money as completing the quest line isn’t very important in terms of completing the DLC successfully. Make sure you save before speaking to the smuggler leader as well as during the process of the glitch just in case you choose the wrong speech option. Please see the video below if you wish to see the information above visually.

I discovered a way of getting unlimited XP in DA:A, which is a good way to 1) Level up your character to get this achievement, and 2) Level up a freshly started (non-imported from DA:O) character high enough to more easily kill the harvester in The Golems of Amgarrak DLC on hard, which is a difficult achievement if you are otherwise trying to use a non-imported from DA:O character. Raptures Lost's solution looks like it would be a faster way of getting XP (I have not verified if it works), but if you 1) Are already past that point in the game by the time you saw it; 2) Don't want to use an exploit like that; or 3) would prefer to idle boost XP, then this is an alternative infinite XP solution.

*Minor Spoilers for Blackmarsh Quest*

What you have to do is go to the Blackmarsh and proceed until you get out of the fade back to reality, and you are tasked with killing several fade portals.

What you need to do is immediately go to "Advanced options" (hold left trigger and bring up the skill wheel) and make it so your group is stationary as soon as you come back to reality. Take your main character and find one of the portals and kill the revenants around it. DO NOT KILL THE PORTAL!

The soul portals are stationary and indefinitely spawn Shades to attack you. These shades are weak and provide 79 XP per kill, which is good towards the beginning of the game and mediocre to poor as you approach level 30.

If you are strong enough, you can leave the controller idle and let the shades keep spawning and coming up to you to attack. Obviously, you will need to be using short range weapons for this to work, so your character does not start attacking the portal. I've left it going for about 20 minutes now while typing this solution and I've gotten over a third of the way from 29 to 30.

-One thing to note is that the Shades sometimes knock you over, which pushes you back (and further away from the portal). Eventually they will push you so far away that they will no longer spawn, so you can only leave this for about 20-30 minutes at a time without checking back and re-positioning yourself.

-Another thing to note is that sometimes the shades will push you in such a way where your guy will not auto attack back. This usually occurs after they push you into a wall. I have not found a way to fix this. If this could be fixed, I see no reason why you can't idle boost overnight and get numerous levels. I tried messing with the tactics of my guy (and switching to another character so the tactics would actually be used) to no avail (The shades will not spawn if the character you are controlling is not close by. If you bring a second character over, the shades will come to that character instead of your main). There may be a way to fix this with items that reduce hositility or spells/skills that make you a less appealing target, but I haven't figured anything out in that regard yet. An alternative is something that prevents your main character from being knocked back.

-I personally tried this with a Rogue with as many dodge % increase items I could find, so it was very difficult for the shades to hit me. If you don't implement this strategy your mileage may vary (they may hit you enough to actually kill you, or you may get knocked back much more often requiring more often position resets). You can also position a mage to heal you if you take away their staff so they won't attack the portal.

-If your main character is ranged, you may be able to get them just at the right distance so they won't shoot the fade portal, but will still allow the shades to spawn. Using this method you could probably idle boost overnight.

Leave a comment and please give feedback to let me know if this is a viable XP boosting method for your character.