Yeah, I agree that Sylvan+Wyngaal is your key to victory. I also suppose you can win with Necro+Deirdre (or Vladimir) but Sylvan victory would be easier.
1) Attack: Archery+Battle Frenzy or Flaming Arrows
(Flaming Arrows is very neat with Triple Ballista. However, if you go for Leadership, you really should pick Battle Frenzy+Retribution.)
2) Luck: Soldiers Luck+Elven Luck
3) War Machines: Ballista+Triple+Imbue
(Sylvan heroes are not offered War machines too often, so take it the first time it is offered. If you miss this, go for the Ultimate and pick Logistics: Familiar Ground, especially if you agree to meet on grass terrain. Another very good (maybe better) alternative is Leadership: War Dancers+Swiftness.)
4) Light Magic: Wrath+Storm Wind+Refined Mana (req for ultimate but if you don't go for the ultimate, I suggest Abjuration, then you can also get Suppress Light and ditch Storm Wind, maybe)
5) Destructive: Ice+Storms+Sap Magic or Cold Death
(Sap Magic is good against divine vengeance, while latter is very good with Imbue(with cold spell)+Triple ballista. With that combination, I suggest imbuing with Lightning Bolt for use against Champions, since the ATB multiplication by 0.7 is cumulative and ATB value will be multiplied by 0.7^3=0.343. However, if you have high knowledge, I suggest Deep Freeze which will do much higher damage).
Overall, there is more than one way to accomplish the task. Although you're not a fan of Leadership, combined with Retribution, it's a killer considering the Swiftness perk. Logistics is suggested only for the Ultimate.
I agree with other suggestions that you should rush to kill first, and Wyngaal will help you a lot in this. Your Sprites, War Dancers and Dragons will most probably attack before the Imperials, maybe all if your friends does not get luckier than you do with the artifacts. Your shooters might attack before the Champions too.
In terms of creature selection, Sprites and War Dancers are a must. I suggest Ancient Treants and Druid Elders but alternatives are just as good. I strongly suggest Crystal Dragons combined with high Luck. Arcane Archers will do better damage especially to high level creatures and have higher (11>10) initiative, but if you get the Unicorn Bow, than Master Hunters ftw. MH might prove better even without the Bow, thanks to their specialty.
Selecting Angels, Champions and Griffins as favorite enemies, you should have a fairly easy time actually...
My 5 cents...
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