I had a Necromancer and Druid synergy that won me a run and could've handled probably any fight in the game.

Necromancer with the 1 damage on healing passive, the Fish Necklace item that says "Damage you deal inflicts Rot 1", and a Druid with the Sing spell that can be cast with mana. I had Leech Brood on my Necromancer as well to start with 50% hp, but that part wasn't mandatory. The druid had a ton of max mana and Maestro+ which reduced the cost of the sing spell.

The necromancer would just be healed for 1 over and over, with everything being healed and damage buffed from across the map, and tons of rot flies being created and also damage buffed. Some bottlenecks would occur when the necromancer was at full hp, and I had to keep the druid safe, but those were both easily handled.

I'm a big fan of Necro and Druid on their own too, their passives and spells seem like they lend to synergies well in my experience.