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I haven't played much with the Elves, mostly going through the "Rebellion" Campaign, and I don't think I've ever played them online. They seemed too weak at the beginning. Of course, I put Centaurs in the front line and developed them further towards the wild ones – they don't have very high initiative, but they do good damage, and the additional damage from critical hits is quite good, although the captains seem decent too. Instead of one Centaur, you could also use a warrior hero, but he's too weak for the initial battles with the Elves, so it's better to use an archer hero; a Dryad is also good, but again, they lack hit points.
Why Centaurs specifically? There's simply no alternative to them – a Griffin doesn't count. Is there any logic other than Elven logic in building their lair? It's expensive, and in the end, you just get a large bird without any special abilities.
Next are the archers. As a rule, the Elves should always have the best archers, and here the selection is the richest, so there's plenty to choose from. I almost always developed a regular scout into a Stinger (the one with poison). As the level increases, the parameters and damage increase, including the poison damage. I rarely took its alternative – in theory, 2 shots are better than one, and the total damage should be higher, but if the archer misses, its effectiveness immediately decreases, and it seems that they can't attack different targets, which is not very good in battle. There's also a branch with elemental archers – high initiative + additional fire or ice damage to choose from, so there's not much difference – one has more damage, and the other has slightly stronger effects.
There's not much to say about the healers – mass healing and then, depending on your preference, either defense for one turn or against fire or air; I always chose based on my mood.
And finally, the mages – they're quite weak, as is the entire Elven army at the start. The only variety is the fork: if a beautiful Archon deals more damage with lightning, then its scruffy-looking wild elf counterpart is a bit tougher and, in addition, reduces the armor of enemies with each lightning bolt. Unfortunately, the city guards are immune to this ability, which is a pity.
By the way, in the expansion pack, I encountered a race of dark elves, and they have a rather interesting campaign. It seems that they either died or fell under the control of Mortis. In general, they are distinguished by a specific arsenal and not very pleasant magic. I had to struggle a bit in the final mission of their campaign – if the paralyzer hits, you can consider the battle lost