My experience tells me Druid is at a big disadvantage vs Warlocks. Sure, Druid has some hydro spells and has new ways to cheaply get rid of Burn tokens (e.g. Extinguish, Raincloud, Meredia's Blessing), but how many is she typically running? Meanwhile, most plants have Flame +X, whereas few demons have Hydro +X (although Battle Forge and Fire Elemental don't want to see a Druid across the table). Also, Warlocks often run Deathlock, and that negates all the Regen that a Druid and her plants depend on. I don't think I've ever seen a Druid win an Arena match against a Warlock when the Warlock has played Deathlock.
Generally speaking,
Plant (Druid) > Water (Siren) > Flame (Warlock) > Plant (Druid)...