Every few seconds, the Grappler will grab the troop that is closest to it, and pulls the troop close, dealing minor damage in the process.
Critters will distract the Grappler, so you can use it to avoid large units from being grabbed.
If Critters are not available, the best strategy would be to use smaller units such as Riflemen or Zookas. The loss of a single Rifleman or Zooka is usually immaterial.
The Grappler should be placed in the middle of a bunch of other defenses. As its use is best when grabbing a large unit such as a Tank, place it in range of multiple Cannons or Boom Cannons to eliminate them quickly.
You can also place it in the middle of a group of Flamethrowers to wipe out other units such as Riflemen or Grenadiers quickly.
The troops that it drags every so often can serve as a distraction to the other defenses, so placement of the Grappler has to be carefully considered.
At Mark I, the Grappler consists of a yellow base piece, supported by a metal platform that is bolted to the ground piece. Behind it there is a simple metal pulley attached to the base piece by a formed wire. The claw of the Grappler consists of two metal prongs.
At Mark II, the base piece and support have been unchanged. The attached pulley has been redesigned, having been elongated and recolored from dull grey to red. The formed wire has been elongated to suit and the claw has seen the addition of a third prong.
At Mark III, the base piece has still yet to change, however the support is now yellow and reinforced with outriggers. The pulley has retained its material from Mark II but has had its end retainers resized. The formed wire is now fitted with a spacer to hold the pulley in place. The three-pronged claw in Mark II is replaced by two wide metal claws with red pads.