The Quest for Truth

I continued playing through the third campaign. I don't know if this is a bug, but the elemental creatures only appear after the main boss's units are defeated, and only in the battle against Sandro. At first, I struggled with these elementals, then I decided to spread out my army with regular Necromancers, and I was quite surprised. I took Lucretia and continued to be pleasantly surprised. Unfortunately, I wasted a good amount of time – with Lucretia and without the elementals, I could have defeated him without all the fuss. Is this intentional?