I use what I call "sticky targetting".

example Chain Lighting
-----------------------------
/assist [noexists] focus
/assist [dead] focus
/assist [noharm] focus
/cast Chain Lightning
-------------------------------

As long as my clone is not targetting something it can attack it will assist focus. However if it can attack something it does not assist. So if I lose my target or change targets the clones will still attack its current target till it dies.

note: it could probably be changed to
/assist [noexists][dead][noharm] focus
but I haven't tested that.