Not sure how it works with ISBoxer, but I just go in and comment out the assist in the macros of whoever I have up there. For example, my Ret's Step 1 macro looks like this -

Code:
/assist party1
/targetenemy [noharm][dead]
#/tar Weak Spot
/castsequence reset=5 Judgment, Crusader Strike, Judgment, Inquisition, Templar's Verdict, Templar's Verdict, Templar's Verdict
When I do this boss, I change the # so it looks like this

Code:
#/assist party1
/targetenemy [noharm][dead]
/tar Weak Spot
/castsequence reset=5 Judgment, Crusader Strike, Judgment, Inquisition, Templar's Verdict, Templar's Verdict, Templar's Verdict