yep you should slow down and learn to target right targets at right time :P will come later with multiboxing experience

And not really, 5-boxers prefer insta casts only because they insta kill most targets or inflict heavy injures.

I have /assist in macros with spells, so no, thats not the problem.

I use /assist in separate keybind only when my warlocks cast drain life/mana and I need to change their target without interrupting spellcasting. Otherwise it seems pretty useless.