Ok. I think I've got it. Thanks a lot. I need to now list out which abilities don't activate the GCD. (trinkets do not, right?)

Thanks again

so ...
/startattack
/castrandom Devastate, Revenge, Shield Bash
/cast Shield Block

... should cast Shield Block whenever its up because the Shield Block does not activate GCD. or does Shield Block have to be placed before the /castrandom?