Avoid using the Elemental Mastery, [Flame Shock], [Lava Burst] combo as your first attack as the burst damage is sure to pull aggro.
Useful if Flame Shock is about to run out: hit Elemental Mastery followed up by Lava burst to get the guaranteed crit. (Remember, as long as the Lava Burst spell is traveling, it will get a crit, even if Flame Shock has worn off.)
Great for when Herosim/Bloodlust is popped.
0500Patch 5.0.4 (28-August-2012): Updated; changed from When activated, your next Lightning Bolt, Chain Lightning or Lava Burst spell becomes an instant cast spell. In addition, your Fire, Frost, and Nature damage is increased by 15% and you gain 20% spell haste for 15 sec.
0300Patch 3.3.0 (08-Dec-2009): This talent now increases spell haste by 15% instead of critical strike chance while active.
0100Hotfix (2009-04-15): "Elemental Mastery will no longer allow a player to cast 2 spells instantly."
0300Patch 3.1.0 (14-Apr-2009): Redesigned. When activated, your next Lightning Bolt, Chain Lightning or Lava Burst spell becomes an instant cast spell. In addition, your Fire, Frost and Nature damage spells have a 15% increased critical strike chance for 15 seconds. Elemental Mastery now shares a cooldown with Nature's Swiftness.
0300Patch 3.0.8 (2009-01-20): Redesigned. Now increases the Shaman's critical strike chance by 20%, and decreases mana cost of Nature, Frost and Fire spells for 30 seconds. Cooldown remains at 3 minutes.
0200Patch 2.3.0 (13-Nov-2007): Elemental Mastery: It is no longer possible to get two consecutive guaranteed critical strikes from using this ability.