In the Inferno campaign, in the third mission, there were definitely phoenixes. I encountered them there. These birds are really tough, although they were only good for a couple of bites for my army (considering their number). Perhaps, among the neutrals, fire elementals are also unpleasant in this regard.
The tactic is as follows: on the 1st turn, I summon the phoenixes, while the NPCs slowly eliminate them, I cast "Pushback" on the broodmothers and *poof, poof* :) If anything, I heal with Restoration. Phoenixes can also heal once, with the "Resurrection" ability for 272 HP and deal 82 damage of initial magic to all nearby enemies :)
P.S. In my opinion, in Heroes 6, phoenixes are more beautiful than in Heroes 5 :)