Even creating a second character gives you an achievement. New characters share everything except skills and level. When u start a new character, you have access to your storage, collectiables are shared and so are quests, bunkers, safehouses, locations. Everything except skills. Note: You won't be able to replay any completed missions. I found that odd.
For this achievement, I waited until I was done with game on first toon.
1. Then I used the new character to go back and get all the collectibles. I'm not quite done, but except to have her to near level 7 if not done by the time I'm done with collectibles, which gives enough to finish a specialization.
2. I chose the Hacker specialization because the second skill that unlocks in that tree is Inquisitive Mind, gives you 50% and 100% extra XP. I went ahead and put an extra skill point into this to get the 100% extra XP.
EZ PZ if u use this new toon doing something like collectibles that you had to do anyway. You'll have to kill stuff along the way and that will boost you as well. Shouldn't be a problem with access to your first toons weapons in storage.