Helios uses Fire spells, as does the minion. Neither has a damage over time spell (i.e. Heckhound).
Helios cheats by casting Hex on you whenever you use a ward or charm.
Go to the Bazaar and buy about 10 Fire Shields. Get the 90% variety if they're available; otherwise get the 85% type.
The strategy is to keep a Fire Shield on yourself at all times. Helios will cast Hex on you, but that's a 30% boost to your 90% reduction. While this is going on, get rid of the minion. Then concentrate on Helios. Heal when needed.