In many ways it's easier once you have completed the campaign with one character and received a couple of exotic weapons. It also helps to buy exotic class armours for other classes off Xur, and stick them in your vault along with your highest light guns from your first character. Why? The game drops new items with a slightly higher light level, up until your into the endgame and it starts to plateau a bit more. But unlike D1 in D2 it calculates yoyr light level based not on what you have equipped, but on the highest light possible from all items in your inventory. So when starting a 2nd or 3rd character, as soon as you reach the Farm dive into the vault and pick out those high end weapons and the exotic class armours. Drop them in your new character's inventory. Yes, you won't be able to use them until you're level 20, but as far as I can tell they still count. Because I burned up to 280 light with my 2nd character long before I'd finished the campaign.