I just attained Astral and Umbral level 3 buffs and notice they give you a wonderful perk of 50% reduced casting time of the opposing element in which you are currently buffed with. Remember, Astral and Umbral get placed on you when the spell you cast DOES DAMAGE, not when it is cast.

So, when I have Umbral III, i tend to be able to get off 2 Fire III's back to back (at 50% cast time) before Astral III kicks in and my casting speed is back to normal on fire spells. After those 2 Fire III's i then can do 2 Blizz III's (at 50% cast time) back to back. I then repeat these steps for some pretty massive damage.
Keep in mind if you are going to try this you need to time out when you get the buffs. doing 2 fast Fire III's is easier cause of the delay on the damage compared to Blizz III almost instant damage after casting. The timing window on both of these spells is pretty small, so SPAM THAT BUTTON! your spell bar WILL glitch out about 80% of the time you cast the 2nd fire III/blizz III.