I totally agree about the instant cast spell to burn the adds. Anything that has a casting time, or even ranged shots, will make the add shoot you and you'll get the debuff (bad arcane dot).

If you're having trouble targeting the adds, like me and everyone in my group, you should make a simple macro that reads:

        /target pure energy

Then all you have to do is spam click your macro as soon as you see the yellow message that says "Vexallus starts to draw pure energy".

You can even integrate your instant cast shot in your macro... we had a dps druid and he took care of all the adds by spamming the following macro:

        /target pure energy
        /cast moonfire

Hope this helps.