Inferno – “A marriage between heaven and hell” [Campaign 5, Mission 4].

You need to defeat the Temple Guard as quickly as possible – after that, nothing will hinder you. Wait for the Temple Guard to “wander around” and capture his city. Then, all that remains is to finish off the Knight – he will be in a Fortress nearby. After that, capture the neutral cities – there are 3 or 4 of them. Then, you shouldn’t have any problems with the Elementals anymore :).