I don't think I understand the question. Does this not work:

/castsequence reset=170 Water Shield, Flametongue Weapon

It kinda depends on how often you are getting hit as to how soon the shield goes away. If it is going away earlier, then you can put in a shorter time. The weapon enchant is a 30 min buff, so would only have to push the button 2 times every 30 mins.