The Impaler is classed as an Individual in the Dwarven list, so it would have all the rules for an individual missile troop.
Normally missile troops cannot fire at a more distant model if there is an unpaniced enemy model within 12".
However there are two exceptions.
1. Individuals, such as the Impaler, can fire at a more distant target, even with an enemy within 12".
2. A model can fire at a larger, more distant target, as long as the firer can see at least 1/2 of the target, and the target is at least 1 SZ larger than all closer units.