Depends on which decks you are the most challenging for you. If you have problems with weather (which I had at some point, mainly against Eredin) its good to put Ida in for Clear Skies (or 15 point swing if you dont have weather problems). Roach could also help a lot to gain tempo (which you of course lose a bit when you take the witchers out).
For me it is mainly try and error, putting some silvers in, keep them as long as they work, otherwise switch them for something else.
For the golds: I think Aglais can be nice, I actually dont have the card, but I think she can be really valuable in certain match ups.
I found myself also struggling with decks that have big finishers in round 3 (like pirate captains, spellaetel) so perhaps Gerald Igni or Borkh could be a wise choice
But again that mainly depends on what you are struggling with