I just had an awesome idea for how to make use of the 900 hp cap for a cool rare NPC encounter... and a Kong badge to go with it:
NPC name: Overloaded Phase Dragons
Stats:
hp = 900
mark multiplier = 1
draw multiplier = x2
Kong Badge: "Riders on the Storm"
(for those who don't know the song reference:
http://en.wikipedia.org/wiki/Riders_on_the_Storm)
To earn the badge, you have to: "Find and take down the 'Overloaded Phase Dragons' before the deck runs out."
Its deck size is 10 cards, which bumps to 20 after the multiplier, so to beat the NPC you only have to survive for about (20-7)/2 = 6 turns... Having tested it out, thats not really too hard as long as you put out some creatures to absorb all the damage spells.
Taking down that many hp before its deck runs out, however, should be a fun challenge (likely requiring some clever deck building).