How Demon Class Works: The one thing that the Demon Class excels at is being both good at AOE and Single Target Attack. The Demon Class has a skill that changes every one of its moves to AOE or Single Target, so AOE does less damage, and Single Target does more.

Why is Demon Class the most Speedrunner Friendly Class (In my opinion): If you have high enough attack, you can use AOE to attack multiple enemies and kill them in a single shot. As said in Sealio's Guide (https://www.speedrun.com/world_zero/guide/tfm6i), grouping enemies together is the best way to kill enemies. This is why Demon Class is good for speedrunners.

While Demon Class has many uses, you will need a dragon pet to help you out. (Psion beam is still good!) As seen in my Wheat Farm Speedrun, you can use Psion beam to kill multiple enemies at once. (https://www.speedrun.com/world_zero/run/zq499n9m)