"Confident" achievement
 

How do I obtain this achievement? What constitues attacking Eric with the fewest allies?

do not recruit demon army as it is optional

thats it?

demon army as a Xeona not demon units

Also for the achievements from earning multiple achievements, do they all have to be earned in one playthrough?

Sorry I don’t know 100% but I think these 2 or 3 achievements you talking about they might be accomplish only on 1 run thru, BUT again not sure.

Yes.

I saw that you can also recruit Iron Richard...how do you do so and will it break my achievement?

Where did you see that?

You need to refuse to kill the rebel leader in Greenwort and warn him of the plot to assassinate him. Later when you return to the rebel hideout after the first Guilford fight Iron Richard will be there and you can recruit him. You won't get the Confident achievement if you do, though.