Pixie Teleport Trick (Heroes of the Feywild page 33)
Daily: Teleportation
Minor Action, Personal
Effect: Until the end of the encounter, whenever an enemy attacks you, you can use an immediate reaction to teleport up to 5 squares.
If there is only one monster that has only one attack, are you essentially invincible?
No. The teleport trick is an immediate reaction, not an immediate interrupt. Since it's a reaction, the entire attack occurs before the pixie can teleport away, and the pixie can still get hit.
For reference: a reaction to a trigger occurs only after the trigger (in this case, an attack) has been completely resolved. An interruption occurs before the trigger is resolved.
(Also, to make matters worse for this exploit, it's a daily power, so you'd only be able to use it in one encounter per day)
