Quests and puzzles!

Everything would be fine, even 5000 mana would be enough to defeat the Lava Dragon, but... if it weren't for the fear they inflict. Missing a turn means the end for the Ifrit. And the slot for summoned creatures for distraction is cleared in one go. In general, things don't work out in practice as well as they do in theory. By the way, you can't force a Lava Dragon to occupy only one slot without a hero...
And also - take a closer look at the Physical Mitigation - the retaliation from the expert shield will not exceed 30% of the damage, i.e., in fact, 5 Lava Dragons, when one hit deals 400 earth damage.