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(Sorry for necro-posting, but just in case it helps somebody). I didn't insta-sell the bridges. But what I did was bridging sideways, from the 'dungeon special' islands directly to the enemy lair. Before the enemy figured out what happened and turned back, my imps have claimed most of the rooms, and my high-level creatures could get to the dungeon heart (but, unfortunately, most of them perished there).

And there comes the tricky part: when the enemy does get back, use the hit-and-run tactics. Deploy some creatures to the dungeon heart. They will start crushing it. As soon as they get spotted - DO NOT let the fight continue and DO NOT try to kill all enemy creatures (most likely, you will fail). Just select your own monsters in the creature tab and move elsewhere (or even leave them hanging in the air, if you are lazy). When enemy forces find no new threats and move back, repeat the sequence again... and again, and again, until the heart is finally crushed and the level is won! That's how I could beat this level after three days of scratching my head - bridges from the sides, hit-and-run at the heart!
Post edited July 03, 2019 by 1AdAstra1
What also work is simply defeat the enemy when he attacks you. To do so, make sure to get several imps from the start, build rooms quickly, get training right away, and while you do that get towards the 'increase level specials' on the sides of the map. (Possession is helpful here).
The enemy keeper has no real advantage, except for the fact that he starts with a training room. If you use the increase level-specials when he is going to attack, you should have the upper hand. This goes doubly so if you bring a unit with you at the start.