My most effective start for the Inferno faction was when Deleth found a tent on the second level; after that, everything was very easy.

It's clear that you need to upgrade the Imps and Jumping Demons as quickly as possible; the next task is to get Succubi and upgrade them to Temptresses. It turns out to be quite good.

Sometimes, when attacking strong archers (e.g., Elven Archers), it's wise to deploy 7 units of 2 Imps each and summon near the archers.
The trick is that the archers will most likely just run away from the summoned Imps, while the ballista does its job. Even if the archers destroy the summoned units, there's a high probability of getting away with only minor losses. And these are deadly Elven Archers! Not every race can defeat them with minimal losses.