Here's what I use on my guys:

# all locks have this as the first line
/assist focus

...then...

LOCK1: /castsequence reset=10 fear,,,
LOCK2: /castsequence reset=10 ,fear,,
LOCK3: /castsequence reset=10 ,,fear,
LOCK4: /castsequence reset=10 ,,,fear

Works like a champ, just have to remember not to spam it.

As you mentioned, it might be the focustarget parameter. Try just doing /target focus and then /targetlasttarget instead.