Tactics for playing as the Sylvan faction.

I usually use a combination of archers and ents because ents provide excellent cover, in my opinion. Also, upgrading the archers allows you to quickly and efficiently defeat high-level creatures with minimal losses (you can attack on Utopia, defeat phoenixes, etc.). I upgrade the archers whenever possible because, as mentioned above, it speeds up the upgrade process. I don't include dancing sprites in my army until the very end because they are weak, and upgrading them is expensive, and you can manage without them anyway.
Elves are an aggressive race, so, in my opinion, the upgrade path should be built with a focus on a blitzkrieg strategy:
1. Attack (choose perks based on the situation; I usually take archery, but not always).
2. Light, IMO, is definitely the way to go. Mass acceleration doesn't need any explanation, and mass smite will boost the elf's not-so-high attack.
3. Luck, without a doubt. You can upgrade to Elven Luck, but again, it depends on the situation; magic resistance might be necessary. After that, choose based on the situation, but I usually take logistics and education; leadership can also be a good choice.