Depending on your loadout, I would advise exploring underground in areas like the ice, corruption/crimson, and hallow to find weapons from there. The mimics typically drop good weapons, especially the hallowed mimic which drops the Daedalus Stormbow, flying knife, as well as the crystal vile shard.
For Armors, I'd recommend looking for sandstorms and then killing sand elementals from them so you can get the Forbidden armor from the fragments it drops. It is ideal for a Magic/summoner hybrid that you're going for. Your friend should aim for the frost golem in blizzards. And you will both need either Titanium or Adamantite ore to complete your set.
Old One's Army is also worth doing since it is at Tier 2 now from your effort killing the Destroyer. You can get some effective weapons from killing the Ogre enemy in the last wave along with a good light pet.
