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Regarding the Nimus vs. Orson matchup. I encountered it quite often. I regularly won with Orson, but I don't think I ever won with Nimus. In the provided replay, Orson wins with a huge advantage; he has mana left at the end of the battle. For the first cast with Orson, I would still use mass haste. In the provided replay, Nimus made an obvious mistake (among others) that decided the outcome of the battle: a phantom on the demons instead of a phantom on the demoness. And the spirits should have been attacked with the magic of the cave lords.
Here's another replay.
Dwarf vs. Helmar: in version 3.1, Helmar will be stronger, I think. But the dragons under the battle rage rune did a lot of damage. They should have been taken out earlier, instead of being left for later.