I had the same issue with my rogue. So I changed the Auto-Assist macro to do /assist [nodead] {FTL}. That way all the dps stay on a target until it is dead. Of course you need a way to change targets, so on my "pull" keybind I use the regular non-auto assist macro.

You can change the auto-assist in the virtual mapped keys section of the character set. See https://isboxer.com/wiki/Virtual_Mapped_Key