I don't think relying on Soulfire for the Spell Damage is a good idea since a random card must be discarded. Also, there's a good chance that Malygos will be instantly killed when you drop it down due to the lack of big cards you are throwing down.

This is why I think the route of Druid/Rogue is more dependable, because they can "clear the way" for a safe Malygos drop by either throwing down big threats before/with Malygos or they can conceal it.