The Elemental Skill Tree is the strongest of the three, offering both strong Wind and Fire Skills. With a large Area of Effect damage skill in Fissure and a close range burst damage from Firestorm, the Fire Druid can dominate in areas without Fire immunes. Unfortunately, many areas in Hell have a lot of Fire Immune monsters dropping this build from S tier to A or B tier.
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In Normal and most of Nightmare, the Fire Druid is extremely strong. The Fire Elemental skills that the Druid possesses do a lot of Damage with high range/area.
It is strongly recommended to boost your mercenary’s Damage. Doom along with an Act 2 Offensive Mercenary (for Might, to boost his and your Grizzly Bear’s Physical Damage) is the best way to do this.