You can also summon Grimson FIRST. As a Legendary creature there can be only one in the arena at a time. Now it becomes his problem to Kill Grimson.

The other solutions posted are more flexible, but this one can be fun to pull once or twice. It really forces the other player to look at response options if the tables were turned. If your opponent is always playing this one strategy it can shake things up.

Love it!

Ok, I'm looking for some advice here on whether there's a low resource way to handle this situation (I can't really think of one).

My opponent plays an Air Wizard.  His opening depends on whether he faces an aggro or a slower caster, but it is built around Grimson Deadeye.  The goal is to get Grimson out, provide him with an archer tower and then to put a wizard tower with lightning bolt down in an adjacent zone to stunlock anyone who goes after Grimson.  Leave him along long enough and he'll also drop walls, throw Hawkeye on Grimson and do other things.

Against an aggro caster, he remains still, drops Grimson on turn one and casts Rouse the Beast.  On turn 2, he'll start putting up his defenses.

Against a slower caster, he will move twice and quickcast an archer's tower, then cast Grimson + Rouse on turn 2.  Turn 3 is the wizard tower.

Assume he has more than one Grimson in his deck.

I'm looking for suggestions on how to counter this which [a] don't require significantly more mana/cards than he invested in building his combo (30 mana, 10 damage and 3 cards is not a good trade for a 15 mana card) and which will work more than once (Reverse Attack, for instance, might work the first time, but after that Grimson won't shoot until after a Seeking Dispell).  Happy to have the thoughts of the community on this...