We ruled that Berric cannot be targetted or affected by Wildfire Assault, because his ability doesn't 'Save' him. Was this correct?

That is indeed correct.

Beric cannot be killed when you have no power on your house. You could actually choose other characters to not be killed and come out with four after the plot resolves.

A character that "cannot be killed" cannot be affected by any effect (targeted or otherwise) that would kill them (unless, of course, some text says otherwise; the eternal caveat).

Hand of Osiris said:

Beric's "cannot be killed" prevents him from being chosen by effect that would kill him. Technically, the characters you choose when Wildfire is revealed are not being chosen to die. Therefore, it would be legal to choose Beric for Wildfire, if you wanted to. That particular plot can target him.

It would be a complete waste, though, because - chosen (to live) or not - the plot cannot kill him. So if you have 4 characters on the board, you could choose Beric as 1 or your 3 (to live) and let the plot kill the 4th. But, as you have certainly figured out, the much smarter play is to choose the other 3 (to live) and let the plot try to kill Beric - and fail.

Another thing to recognize here is that just because Beric cannot be killed, and Wildfire chooses characers to libe, you are not required to choose Beric as 1 or your 3 to live. Many people make this mistake, thinking that you must choose a "cannot be killed" character to avoid Wildfire. That is not the case. As outlined above, while it is legal to choose your "cannot be killed" character when Wildfire is revealed, it is not necessary.

Thanks! with more characters