/castsequence scorch, scorch, fireball, fireball, fire blast, pyroblast, fireball, fireball, fireball, scorch, fire blast, pyroblast, fireball, fireball, fireball, scorch, fire blast, pyroblast, fireball, fireball, fireball, scorch, fire blast, pyroblast,

Is what I currently use. Works ok for most situations with a 50% crit rate and Hotstreak. Pyroblasts end up with a 90% chance of being instant. I have no idea how Arcane works though, so I can't offer any advice there.