I use a system that only "assists" when the current target is either friendly, dead or empty (i.e., no target). This way I can mash an attack button and my DPS will continually attack their current target. Once dead, they then look to assist "party1" to acquire the next target. I like this as it allows my main (Paladin) to change targets freely without my DPS following. If I do want to switch targets mid-fight, I simply clear the current target for all windows (i.e., ESC) and resume mashing my DPS button.

Example, my frost mage primary DPS macro:

Code:
#showtooltip frostbolt
/use 13
/use 14
/cast icy veins
/castrandom [exists,harm,nodead] frostbolt, summon water elemental
/stopmacro [exists,harm,nodead]
/assist party1
/castrandom [exists,harm,nodead] frostbolt, summon water elemental
From the video it looks like I either a) have to set up a discreet assist button or b) embed an assist function without the harm/exists/nodead check.

Without testing, would it work if I put the first six lines of the above macro in the macro window and then added a checkbox to the "but first" box?