I wrote this somewhere else recently, but in ISBoxer you can make a new auto-assist macro and change your team's standard auto-assist me keymap to use this one instead.

/stopmacro [harm,nodead]
/assist {FTL}

It means the assist won't change target until the first mob is dead. I use this myself for combo point classes in Classic because otherwise they would lose cps when switching target, but you can use it also in the case you describe. It's also useful to have a "pull" button that does use the regular assist macro, so that you can explicitly change targets if need be.