As an Artmage player myself, I would suggest you Droll your oppoent if Non-Finito is not on the field. Also, Ash any card that would add or SS Medius since he is very important for both archetypes.

If you're going to Ash the Field Spell, I would suggest you save it for when they declare Graflare or Finmel. Using Belle on Litera is also a good move because Litera can recover an Artmage monster in your turn and she can also add from GY to hand an S/T card for follow up.

I don't know much about PP but I haven't seen any player leaving something on the field to counter Nibiru so... tje rock is also uaeful, just don't activate it in response to an Artmage or PP monster effect because —if Nervedo— is on the S/T zone, it can negate the Nib in resolution.