My 7th slot was a second Assassin for the purposes of boss rushing. However, given that you can pretty much boss rush with most classes, you're more likely to make the decision based on how lucky you have been with getting skills.

One key aspect of having a second class is the fact that you can't use the same skills on the second hero. So keep that in mind when you decide. I went with Assassin because they are very good damage and can be built for two different purposes (ATK DMG and Spell DMG).

However, if you don't have enough skills to split between two heroes you won't get as much value from the duplicate class.