How to win Here be Demon without our own Druid, please refer to the link below:
Not sure why you care about beating Demon without druid, but a few problems with your assertion. Reduces damage is one of the easier demons. Additionally, your video starts at a point where about half the damage has already been done (read: you previously failed). And, of course, you are only LR18. Presumably, identical demons are easier at lower levels of LR, just as is the case for identical spawns of mobs.
In all honesty at this point in the game it's not very impressive and sort of unnecessary since you should beat it anyways with or without him, really isn't worth the risk of it's just for bragging but if there were a practical way to go about it, like using a santa or immo or sz or vlad or some high damage dealer instead while still healthily Maintaining everyone's health, then it would be worth it. But either way, only changes things slightly for higher risk