One potential way around this is to lure the guard outside with a well-placed knock, then run inside while he is not looking. If there are locked doors you can also locate the guard carrying the key and lure him outside to steal it. Also play around with leaping under and out from tables. If you can arrange for the guard to only see you for one turn you can get by him.
You will especially need these techniques when you get to the last couple of levels.