Tactics for playing as the Sylvan faction.

We're not worried about the fire elemental in the late game; we're more concerned about rushing it.
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with minimal magic, elves are weak

Elves have high knowledge and low magic; this is precisely what senior druids exploit. Knowledge remains high, while magic is further increased and becomes comparable to that of a Sorceress or Necromancer. And again: chaos is not effective in the late game (especially against a mage), and light only needs magic in the case of a few spells, so the main advantage of senior druids is increased damage, as antonag07 correctly pointed out.