Since this is now about bosses in general, I think that slightly reworking the mechanics is way better than reworking the numbers. Some numbers should get reworked a bit, like Skarrik’s isAlive(); timer, but overall they’re good enough.
That doesn’t mean buff Skarrik’s HP, but perhaps give him a crit resistance or an ‘ability resistance’ or even more simply a ‘damage capped per 5 seconds’ that would make GK, BH, Shade and such not one-shot him.
And since it is relevant, let me recycle an old post.