My Mage is at level 18 and is half way to level 19. I am on the final onslaught, just after dealing with Loghain. 
 
Can I make it to level 20 (achievement) just by completely the rest of the game or do I need to go XP hunting? 
 
The final section appears to be pretty long with a lot of fighting but the darkspawn are crazy easy and Im assuming down provide good experience.  
 
Thanks for the help    

You didn't need to make a duplicate thread.  The mods would have moved your old one.
 
But, while it is possible you might hit in the very last battle (or maybe a little sooner if you COMPLETELY clear out the castle), I don't think it's very likely.

Yes, but you need to disable every trap, pick up pretty much every Codex-thingy and so forth. The DLC brought me up to level 21 and I am very close to 22, just waiting for the new DLC to be released because I literally ran out of game.

Yes you can there are so many darkspawn to kill that you will easily gain the last levels.

Oh true, I forgot about downloadable content. I might invest in that, we'll see.

Warden's Keep is pretty nifty since you get that storage chest for the campsite.

Which one would probably provide the most experience?

I ended up making it to level 20 at the fort drakon section. Cut it close but I got the achievement.