My fire mage has two castsequence macros. One for attacking enemies that have more than about 8 seconds to live, and one for when the enemy is going to die soon. The one for enemies that have a while to live includes fireball, pyroblast, living bomb, scorch, and fire blast. The one for enemies that are about to die includes just scorch and fire blast. I also used a little "double comma" trick to make sure that a fireblast cooldown from one castsequence macro doesn't lock the other macro when it hits fireblast.

Example of the double comma thing:

/castsequence scorch,,scorch,scorch
/cast fire blast

When it hits the double comma, it tries to cast fire blast. If fire blast isn't ready, it moves on.