[Edi]

The way to use a Phoenix for early expansion is to boost it to high fire or air magic, research evocation up to whatever you need and then go to town within own dominion. Just fire darts will seriously ruin the day for any indies that have a significant number of units without shields (archers, slingers, some militia etc). Stand back and keep raining fire and you'll get results. Best send the phoenix naked, since it will be killed.

The immortal's afflictions will disappear over time, that is one function of immortality.