This should work, i think.
Depending on how you have your macros set up, you might need one for each target character.

/castsequence [target=whatever you use] reset=15 Powerword: Shield, Renew, Renew, Renew

etc... or however many times you might want to cast renew if you cant cast the shield yet.
And if you have the talent that reduces the weakened debuff's timer you'll have to set whatever time it lasts as the reset time.
If you have one of this macro for each of your party members, or however you have it set up, it should be able to do what you asked for.