Basically as far as balancing the AI's go, there are 2 issues which should be addressed separately.
The first is the difficulty of the opponents. For this, each level should be just difficult enough that by the time you more or less mastered one level, the next one will provide a decent challenge. As things are set up now with the new AI3's, I think the difficulty is just about right. The first couple levels are easy enough for a new player to win using the starter deck with at most minor modifications, AI 2 requires at least a somewhat decent deck, AI3 needs a solid unupgraded deck, and 5 and 6 need a very well built deck and at least a few upped cards to do well. Also, the quest system is there to basically walk people through the needed steps early on.
Once that is settled, the second issue is balancing the risk vs. reward for each difficulty. Again, ideally each level's reward should be balanced so that you can get the cards you need in order to move on to the next level within a reasonable period of time while also making it so that one you are able to move on, it will be more profitable for you to do so rather than continuing to farm lower levels. I think this is where some rebalancing may be needed now with the new AI3's being harder. If new players will be forced to spend a bit longer on AI2 since AI3 is harder now, the rewards for these 2 levels at least should be increased slightly to compensate. It is also worth noting however that the new AI3's also seem to be a little bit better as far as allowing you to farm rares from them(both giving an increased reward and helping to prepare for the future changes in top50)
As for preventing more experienced players from abusing things if AI3 rewards are increased, aside from the suggestions already made, I have two ideas. First, rather than raising the electrum rewards for lower difficulties, instead simply lower the costs of buying cards from the bazaar. This will make it easier for new players to get started and get multiple decks to keep things from being too boring while grinding early on, however upgrading cards(which affects experienced players more) would still take the same amount of time, thus making it worthwhile to move on to higher levels once you can do so.
Secondly, if you want to go with the suggestion someone made earlier of providing increased rewards in the beginning to new payers, rather than making this time or score based, why not simply provide the increased rewards until the player all of the quests(or at least up until a certain point in the quest list). Again, by the time a player reaches the point of being able to beat at least one fg, they're probably ready to move up to farming at least AI3 or even 5 and maybe top50, so the higher rewards aren't needed as much anymore.
