HOW TO BEAT ELEMENTAL WIZARD AND ITSUKI? ON EXTREME MODE
For the Elemental Wizard I went for a sniper Rifle but you can do this with every weapon in the game, just keep running around and hit the palm of his hands while charging or while he's attacking you, you'll be able to stun him that way, just keep the process until he's dead.
As for Itsuki? as you can see from the video a sniper rifle is definitely not the best choice, Dual Handgun or Assault Rifle will make your life way easier for sure.
For a detailed walkthrough check the video below: