HP burns from Rare champs do less damage than from Epic or Legendary champs, this could be what you're seeing. For example in Clan Boss, Rare champs HP Burn does 50,000 but Epic and Legendary Champs HP Burn does 75,000

Thanks. I am using Ultimate Galek so an Epic. I've also tried him on Spider and the HP Burn is only applied once whenever the Spider takes its turn. There is not the constant application of damage that I see on HH's video. Just strange. 

Hmm I haven't seen the video, so not 100% sure, but may just appear that way due to editing. HP Burns only do damage at the start of the champ's turn. 

"While the debuff is active and at the start of the affected Champion’s turn, they and all their allies take damage equal to 3% of their respective MAX HP. There can be only one [HP Burn] debuff active of a Champion at a time. However, if multiple Champions on the Team are debuffed with [HP Burn], all of these debuffs work as normal and apply their full damage."

It sounds like you aren't applying the HP burn to all the spiderlings like you need to - which requires using UGalek's third ability, but only after somebody has put increase attack on him. His first and second ability also put HP burns, but only on one target. 

The thing is that when fighting multiple enemies, the HP burn procs on any enemy will do damage to all enemies. On Clan Boss of course you wouldn't notice since he's always alone, but on Spider, it means that if there is a HP burn on most of the spiderlings, the big spider will take damage on every turn of a spiderling that has HP burn. That's what you see in HH's video. But it requires having HP burn on all/most spiderlings, not only on the big spider. 

Ah, thanks L9753. I will take more notice of what happens. Much appreciated.