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How is this happening in the Dungeon?
It's strange, I seem to remember that he shouldn't be going down into the Dungeon, but rather running around endlessly on his territory above.
To summon elementals, you need the basics of summoning magic and strong summoning magic (level 2). To summon a phoenix, you need skilled summoning magic (one more level). It's also good to have a "Lord of Spells" in the summoning skill (it strengthens the summoning of the phoenix and elementals).
For a normal playthrough of this mission, Fidaen should be around level 28-31.
In general, I think the ideal build is Chaos + War Machines. To achieve this, I caught war machines in a nearby witch's hut in the very first mission for Fidaen.