Ratatoskr is fine from what I tested him on Beta – what makes the S3 squirrel balanced is the fact that he only affects current green tiles in the board.
Subsequent tiles are not buffed (on offense) or debuffed (against him on defense).
That means the hero actually requires brains to use; you can’t just willy-nilly use his skill at any given time. You have to time it for when you’ve set up a good board to your liking.
Also the fact that Ratatoskr affects green tiles when fighting him on defense, means that his only real good position is as a flank to a blue tank like Richard – otherwise noone should be bringing green against him.