For all my DPS macros I use something like this (I do not use a focus targeting system):

/cast [exists,harm,nodead] DPS Spell
/stopmacro [exists,harm,nodead]
/assist party1

This will assist the warrior's first target. They will keep that target allowing you to tab around. When that target dies they will assist party1 again switching to whoever the warrior it targeting at that moment.

And also have an
/assist party1
bound to switch the dps targets manually.

*I'm at work so these may not be exactly correct. I'll will check when I get home.

I hope this helps.