I hate....really hate... boss actors with super high hit points.
That is the result of a lazy or uncreative designer.
This may have been the only way to make hard fights years ago in the "bit" days; but smashing the attack button and just barely staying alive until the boss dies is not necessary any more (and was never fun ...for myself anyway).
Instead make the boss hard by giving them unusual abilities or advantages that hamper the player's advanced abilities.
In my Oblivion Duke Patrick's Hammer-Blade mod:
http://www.youtube.com/watch?v=Q-d6eS4159c
I had an legendary fighter that you had to fight to win the quest. He was programmed for extreme evasion (dodging) and had a real life combat trick that would put his shield in your face so (for a few seconds) you could not see anything in your field of view except for his shield.
Plus he had a sword that could destroy your shield (later after the fight you could repair it).
For your case consider theses ideas:
Increase the speed of the actor as if they had superior reflexes.
Increase the stagger ability of the actor as if they had more strength.
increase the boss critical hit ability as if they had great skill.
If you want to "scale" your boss you can do things that DECREASE the player's "strengths" by a % so that the effect is a scaled difficulty.
Decreases the player's block skill 50% or 75% during the boss fight as if the other actor was so skilled they could use feints and other tricks to get by the player's defense.
Decrease the player's speed or stamina regen by %.
Decrease the player's Critical hit rate by %.
This can be done by placing "invisible" spells temporarily on the player.
So if the player has a 100 block their block is now 50, if they had a 50 it is now a 25. So this makes the boss TWICE as difficult no matter what level the player's abilities are at the time.