You might try a multibar button 1 and 2 with the following

Bar one
Shadow Word: Pain, Smite, Smite, Smite, Smite, Smite, Smite, Smite, Smite, Smite

Bar two [target=toonA] !Power Word: Shield


Then add in bar 3 toon B, Bar 4 toon C etc.

I do not know if this would work or not though to be honest, as I never tried.

/Click MultiBarRightButton1
/Click MultiBarRightButton2

MB1 would be...

/castsequence reset=target/18 Shadow Word: Pain, Smite, Smite, Smite, Smite, Smite, Smite, Smite, Smite, Smite

MB2 would be

/cast [target=toonA] !Power Word: Shield


etc. etc.

Good luck as this is an interesting idea.

Stephen