This is a fascinating bug. Not only is it impossible to select or control the Grunt at all, he cannot be even interacted with.
I tested a bunch of things, and I discovered that Gruntz can walk UNDER him and they cannot attack him, even with MPTRAITOR on and with any ranged weapons, including magic wandz.
As far as I discovered, the ONLY way to make the Toob Grunt actually react to something is to use a TileSecretTrigger to change the tile under him, and even THEN you cannot control or select him. You can destroy his toob with spikez, but you still won't be able to select him. Even rolling ballz just roll under him if directed at him while he's standing on solid ground (courtesy of TileSecretTrigger).
At most, you can kill him if you use a TileSecretTrigger to change the tile under him into something that would kill/damage/drown any other grunt.
The weirdest part is that it only affects Toob Gruntz whose STARTING position is on a crumbling bridge. Gruntz who use a wormhole to get teleported onto a crumbling bridge won't be affected.
I guess the only solution here is to not put Toob Gruntz on crumbling bridgez. What purpose would it have, anyway?