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Everything written below has been confirmed by hundreds of battles in the Ubicom :) .
1) Dwarves have a 95% chance of defeating Ossir. Their thick armor, resurrection runes, and three stacks of shooters will definitely take out the elven archers, and unicorns and treants are just not a serious threat.
2) Sinister has an approximately 80% chance of defeating Nimue. There are two options here: a) if the demons haven't spread across the battlefield yet, then an enhanced meteor shower will decide the outcome (and if you're lucky... :)); b) if all the hydras are still alive, then, if possible, after harassing Nimue's troops with your own units, cast an enhanced Armageddon. All the demons are finished - 2-3 hydras will definitely remain.
My favorite strategy against Nimue is as follows: take Helmar; position the troops so that two stacks of mages cover two stacks of dwarven warriors in the front, with dragons on top/bottom and on the bottom/top edge of the battlefield. In the first turn, place two fire walls (with the hero and a stack of mages) in front of the warriors and the dragon, so that the enemy can only attack by entering the inferno. Then, either add more fire walls or use implosion/fireball, attacking only the non-summoned units, and don't forget to shoot with the mages to apply the fire mark. Each turn, use runes to restore mana (runic recall).
Nimue's troops will either stupidly huddle together, watching as they are gradually taken out by shooters and a hero-caster with almost infinite mana (runic recall), or they will rush into the fire walls and heroically die :) . Fortunately, dwarves are good tanks, and they can't be taken out quickly.
I have tested this tactic in many battles, and it works 100%, only the battles lasting 10-15 minutes are tiring.
3) Sinister has a 95% chance of defeating Dwarves. The tactic is the same as against Nimue. And resurrection runes won't help the dwarves either :cool:
4) Iris will 100% defeat Helmar in the hands of a skilled player. Griffons will attack the dwarven mages from the air, and the main troops (of which there are very many) will deal with his melee units. The 35% magic resistance of Iris is very helpful here.
Also, a professional can defeat Helmar by playing as Ibbu (Imbu :) ).
5) Sinister will 100% defeat Oran. With all the troops except golems (which we carefully keep in the rear), we try to inflict maximum damage. We use the Hive to take out the dragons (with implosion/frost nova) - they are the only ones who can quickly reach the golems - and then we cast Armageddon until we're blue in the face, because the rest of the warlock's troops are too slow, and Oran simply flies through the initiative scale.
6) With sufficient skill, Anwen can defeat Deirdre. In the very first turn, enchant the hero's arrows with a crystal of secrets and use a rain of arrows - the result: two stacks of undead are completely immobilized. Then, with your own troops, focus attacks on the remaining stacks of the enemy (preferably by overwhelming individual units :) ). No berserks or mind control will be able to do much harm.
P.S. To be continued.