(Dungeon + Necropolis) vs (Sylvan + Orcs)

It's a pretty good alliance because Necromancers need a lot of mercury, while Dark Elves don't need it much (mostly for the guild), and vice versa: Dark Elves need a lot of crystals, while Necromancers mostly need them for the guild, so these two races can provide each other with resources. Also, the Orc will have to think carefully about which resistance to develop, as both magics (Chaos and Darkness) are quite dangerous for them, especially since the best specialists in these areas (Dark Elves and Necromancers) will be casting them. I can't offer any tactics for playing as Necromancers because I don't have much experience with that race, but I do have experience playing as Dark Elves. As for magic, I think it's better to develop Chaos because it's convenient for dispelling the Orcs' rage or taking out Sylvans' archers. Also, definitely develop Luck and Enchantment (from this line, I recommend taking skills that reduce mana costs), and Education (from there, only Magic Attraction, and the skill itself is quite useful), and Attack (from there, you can take Mass Haste, but this depends on the situation; if the enemy immediately attacks, it's better to use Chaos right away). All of this is just my opinion, so there may be other options.

Added 2 minutes later
And the skills Arcane Wave and Sense Element are also essential.