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.

I also consider Assasin a good pick for the reasons stated above, that said i would focus on something that let me farm ruins to make a second ruins group, they take a ton of time so it's always good to have a second team.
I guess a priest helps but her ai is so bad she regularly gets stuck killing some mob while your dps goes alone to face the boss...
A debuff wizard could also help since knight can self sustain with flat dmg reduction + armor if the mobs have attack down, haven't tested but if the debuffs stack even warrior + wizard should work on the boss with proper accesories.

arent 2 of the 7 characters wizards but different races. one just superior?

I think going for fast characters make sense. Just be careful with what skills remain for each class, once your main characters are set up.

I personally can't imagine running a 2nd warrior because there's only one whirlwind available. But that's just me.