Well, he's unlikely to be looking up at the roof unless he's taken a sudden interest at stargazing, so take a peek, if all looks clear have Piecewise do the same and if all looks clear from his end as well, then he can come catch us when we jump. Then we put the grate back in its place and get out of there ASAP.
If the area is not clear however, then this is more of a problem. We've got to move now, so we don't have time for an elaborate distraction. We can try for a simple distraction or go for a Bavarian fire drill but that might backfire and just draw more guards here. It might be easier to just take him out. Too bad we don't have birdwise available.
If there's a single guard, then if he has something that's easy for a rat to steal (like a beret) throw our most not-undead-looking rat at him, have it steal the thing and run away with it. Hopefully that will distract the guard long enough for us to get away with the help of Piecewise. If he calls for assistance, then have the rat head towards the front group of guards to stop them from reaching the back.
Else, if the above plan won't work and he's close enough to jump on, just order our body to jump on him and take him out to take advantage of our super-precision mode. Then we can just drag him in the sewer with the help of Piecewise.
If there's more than one guard... I dunno, I guess we'll have to wait and see how they're arranged to think of something.