My suggestion :
AI5 (Half Bloods) : Make them have fully upgraded decks. Let them draw two cards per turn. Limit them to 2x mark, and 100 health. Keep their decks the way it is now.
Minimum amount of work for the developer, makes farming reasonably easy, and won't mess with false gods. It also provides access to all upgraded cards instead of only the ones gods actually use, so less selling of extra copies for electrum and more useful,new cards won.
AI6 (False Gods) : Implement the False God Realms suggestion with the changes to FG decks I suggested. Let them keep their current advantages. Increase the number of spins here to 5.
http://elementscommunity.org/forum/index.php/topic,11606.msg167263#msg167263 (
http://elementscommunity.org/forum/index.php/topic,11606.msg167263#msg167263)
This would make False Gods much more farmable than now, but they'd still be a challenge, and you'd still need to be a very good player to build good anti FG decks on your own. If you can't do that, you'll stick to AI5 instead and lose out on the extra spins, and better card win chance (more copies of the same card in the FG deck than the HB one means more cards won per spin)
AI7 : unnecessary if other challenging quests are added, like this one below :
Implement a "Get a Challenge" button in the Quest window. This would give you a random quest of "Beat -opponent name- by condition". Opponent name would be the name of an AI3, HB or FG based on player score. The next time within that day the player fights that AI level, that opponent will be playing against him. If he clears the challenge he is rewarded with a card of his choice (AI3 = rare, AI5 =upgraded non-rare, AI6 = upgraded rare or nymph).
The conditions could be
"using a mono <element> deck"
"without healing"
"without playing creatures"
"3 times in a row" (this'd make you fight that opponent 3 times, or until you lose and fail the quest)
"without playing a shield"
"with elemental masterity"
"without destroying a permanent"
"without killing a creature"
...etc, there are tons of possibilities here.
Something like this would result in a near infinite possible challenges, which promotes deckbuilding on your own instead of the current "find strongest deck on forum" solution most people use. It would also have good enough rewards that make it worth the effort, and it would be FUN to do as well.
Make it repeatable, but disable it for 24 hours of after the third failed attempt of the same day.
