You can't really farm exp.... once you complete a mission the first time it does not reward completion exp the subsequent times. If you want to level you Titan do some strikes on the vanguard list or hit the crucible.

You can't replay missions for XP sadly. Which is why the first time you complete them it's best to do it on the hardest setting. Crucible can be a quick way to level, but getting 20 the fastest is done through bounties and doing the story missions. Outside of that once you unlock the vanguard playlist you could just run the highest strike for your level, though I haven't personally tested what EXP they give out (by the time I started running strikes it was for the rewards more than exp).