Today playing Magmar I found a spell you can actually stack for attack power. I foget the name but basically it's +1 attack for 250 mana. Also it occured to me there is a workaround for this bug if you have some powerful enviormental damage spells. You just need to inspect enemy armies and whenever you see a enemy Hero with some ridiculous level or attack power.. indirectly confirming that he's Burning Blade bugged....blast him before he gets into combat with any of yours.
Not a great fix but in my case I had just survived a 50 turn onslaught of attacks at my main city, it was late in the game and with offensive moves against all my enemies I had like 7 cities . Thing were looking up because my power ranking had shot up and people were willing to make peace as they had their armies smashed against me. I was excited to turn it all around and win and suddenly Magmar shows up at level 19 and one shots everthing. So I retired.
I could have pressed on taken some injuries on Heroes and kept killing him and taking his cities since I had stock of mana and spells. Would have took some work spamming pioneers and increasing my area of influence. Just was too new to even consider this.
And once you'd kill him maybe the enchantments would need to be recast.