Dungeons 3

This is done in the "The Storming of Dollaran" level. You need to finish this in less than 40 minutes, and it's a lot harder than it sounds. You'll most likely have to try the level several times before you know exactly what needs to be done to win in less than 40 minutes.

When your creatures are near Crowley, he advances; if Heroes are near him, he retreats. Otherwise he stays put unless you activate his skill which makes him move independently for a short distance until the next skill cooldown. To win, you must escort him all the way up to the north-east through the 2nd of 2 gates. Once he crashes through the 2nd gate, the level is complete.

To do this under 40 minutes, you need to know exactly what you want to build and get it done quickly, otherwise it's highly unlikely you'll get it done in under 40 minutes.

The overall strategy is to spend the first 25-28 minutes ignoring Crowley, except when he's being attacked, then you'll need to send forces to the surface to defend him. Otherwise you need to quickly focus on getting the following things up and running while defending Crowley and dungeon invaders:

1 - Researching the bare minimum to build Workshops for crates. 2-3 is sufficient. Make sure to create enough floor space to hold crates.
2 - Researching the Arcanium to build Mana Shrines (each Shrine requires 8 crates), as well as Mana Storage research if you can, although you could just build more Arcanium floors to increase your Mana storage limit. 3-4 Shrines recommended.
3 - Unlocking and building an army of Arachnids and only Arachnids. 10-15 of them.
4 - Building a Library to unlock Arachnid Chitin Armour research and of course researching it. 2-4 rostrums for the Library.
5 - Researching the 3 minimum spells of Demon Strength, Demon Portal and Meteor Shower. If you have spare gold and Evilness, feel free to upgrade your Demons.

While building your Arachnid army, make sure to use whatever Arachnids you have praying at the Mana Shrines to build up your Mana supply as you're going to need it. You'll also want only Arachnids, as they are tough and good ranged attackers, and the best part is if they die they'll respawn at their leveled up states in your dungeon's Vortex rooms as long as you have Mana. Since you need Mana to cast spells as well as respawn dead Arachnids, those Mana Shrines are your main build goal. I suggest about 1,000 Mana to spam Demon Strength and Meteor Shower and the occasional Demon Portal to return to deal with dungeon invaders.

Once you have 10 or so (more or less will depend on your skill) Arachnids and about 1000 Mana, check your time. If you're near or just past the 30 minute mark, then you still have a chance to get it done under 40 minutes. Begin the assault and push Crowley, using your spells and saving Crowley's skill for when necessary. Push all the way to a point in the middle of the map where it's a narrow path and the game says it's an ambush.

Put one creature at the shown location so that Crowley can move a bit forward. Take a detour to the south with your army to avoid the ambush. Then position a creature at the position shown.

Use Crowley's skill (hit then to highlight his skill and activate it), which makes him move a short distance on his own. One activation is enough to bridge him across the ambush and reach the other end then continue moving with your army.

Once you're at the gate, break through it and fight off all defenders except the group guarding the 2nd gate. At this point you will want to attempt Spawner Killer and destroy the last 2 camps on either side of the courtyard if you haven't already gotten the trophy. Then escort Crowley a bit further to the centre of the courtyard as shown. Remember to liberally use Demon Strength and Meteor Shower.

If you've gotten Crowley to the right place shown above, activate his skill (which would have cooled down by now), which will force him to move forward just enough to crash through the 2nd and last gate, ending the level.

During the entire journey, you may have to return to defend your dungeon: use the Demon Gate spell for this then return quickly by dropping your army back into the portal spawned inside your dungeon.

This trophy is tough but possible. Here's a screenshot of me doing it in just under 32 minutes when I got the trophy, using the method I described.