Hi BoBWiz thank you for your comment ! I didn't think this deck would gain attractivity so I let it as a draft until testing in casu games. But since it gains some likes and comments I will adjust it a little and write card comments to highlight the strat.
I totally agree with you that
Cast Down isn't sufficient to answer all threats (even Ghalta would walk freely on our face). I'm not a grand fan of
Vicious Offering tho because if Niv-Mizzet or Lyra is buffed by any mean it can't kill them. I prefer
Conclave Tribunal that prevent to feed graveyard and also answer PW and enchants.
Vona's hunger already cost some games to my opponents so I won't fetch it in this deck ^^.
Walk the plank is another option considering the mana-base (there isn't a lot of Merfolk out of
Jadelight Ranger and some unblockable blues), or
Severed Strands which I like a lot.
Duress is indeed a more reliable (and cheap) discard than
Arterial Flow but
Arterial Flow can totally empty your opponent hand while stealing some life points... I would give it a try before removing it ^^.