I know they try to balance it out with low Attack and Semi-Low Health and a High Cost, but it still seems the card provides too much control once it hits the field. Normally, you could make decks to run around this error but not in this game since the 'decks' are random (which I like perfectly fine). But, it seems if you do not have the Cheaper Costed Direct Damage Dealers and/or Power Generators in your deck, at least enough of them, it seems one can be too easily locked out of a game. The card is so hard to deal with and when combined with other powerful cards (i.e. Mass Damage Dealers or Power Droppers) one is at a loss. I don't think any card should be made that can present too easily of a lockdown on the game.

What do others think?

   There are more than a couple of spells that cost less mana and take care of it with ease as you pointed out, but assuming you didn't get them, or forgot to keep mana as a counter there are a few things to keep in mind.  there is NO water boost card, so you know you won't be seeing it for at least 6 turns, meaning you can come up with "sub-optimum" solutions if he's starting to get close on water mana... for instance if I have flamewave (not enough to kill it in one pass, I try to keep enough to cast ice golem (gauranteed damage and can't be removed by the AG) so that it dies in a couple of turns and leaves me with a board advantage.  If you see your opponent saving water and think he can play AG, force his hand, leave him sitting there having to choose between healing his AG and keeping you down on mana, or cleaning up the 4 other cards that are pounding him every turn he doesn't block or kill them.  You should have a mana advantage in the sense that he doesn't WANT to spend that water till it hits 12, exploit it by rushing into an elemental or seizing board advantage. forest sprites lightning clouds and hydras are all handy if he's got something out there already to put it counter to, as they get extra damage on the AG, and can help keep pressure on to do something other than heal/defend it. I've actually only got about a 40/60 win ratio against human players when I play the astral guard, so while  I know it works for some people, I've seen a lot of clever strategies to worm out of it.   And plenty more where I have to cash in my water mana long before I can even play an AG.

My best suggestion on how to combat astral guard is to try and use it yourself.  I think you'll find that you can't just toss it out onto the board when you have 12+ water.  Try and use astral guard against the archmage.  Watching the archmage deal with astral guard provided me with alot of insight into the card because he's so good combating it.  After that I convinced myself that the card was balanced.