Dungeons 4: If Looks Could Kill on "Extremely difficult" in 19:48
This level is not difficult unless you want to avoid losing room tiles in which case you need to be quite aggressive about making Gobblers for the first mirror: I recommend using "Create Resources" to boost your Gobbler production to make up any deficit. You can easily beat the level before needing to buy the 2nd mirror (which requires a random resource).
Also this level is a great example of Undead Sucking. You actually start with a significant investment into the Undead Faction, and start with several lvl4 Undead. In spite of this, if you actually try using an Undead army, you'll find progress very slow and tedious, compared with just hiring like 10 Goblins, because the Goblins do a lot more damage especially to armored targets and level up a lot faster doing even more damage, not to mention they cost way less gold to hire, and there's not that much easy gold on this level.
You can keep the Undead they'll even stay alive pretty okay with Horde units tanking for them.