Like all Prototype Defenses, it will only last for a certain amount of time after it is placed on your base.
Every few seconds, the Grappler will fire a grappling hook at the nearest troop, pulling it right next to it while dealing minor damage.
In order to stop the Grappler from grabbing troops already next to it, the Grappler has a large blind spot right next to it.
Higher level Grapplers do not have their damage changed, but have their attack frequency increased, allowing them to grab targets more often.
Critters will distract the Grappler, so you can use them 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.
Riflemen will also be able to attack the Grappler safely due to its blind spot, making it useless.
When running large units, such as Tanks or Scorchers on the battlefield, be sure to put down a smoke on the base of the Grappler. This will keep pulled troops safe until the rest of the attack force reaches the Grappler.
When using Medics, be ready for the Medics to follow the grabbed unit to the Grappler and smoke them a trail to follow the pulled units. Note this only works with beach bases.
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.
Alternatively, a difficult and high risk strategy against Tanks and Medics is to place mines and relatively few defenses nearby. The damaged Tank triggers medics to attempt to reach the Tank and heal it, allowing other defenses to pick off the medics, potentially crippling a Tank and Medic attack.
Attackers could potentially counter this by distracting it with Critters, or even using a Medkit on the Tank, so that the Medics will not be pulled to heal the dragged Tank(s). However, if the Tank(s) that are pulled survive long enough to destroy the Grappler, it could still split the attack in two, forcing the attacker to use a Flare to regroup the troops (which could allow defenses to pick off the Medics and Tanks) or simply leave the Tanks at the mercy of the defenses.
You can also place it in the middle of a group of Flamethrowers to wipe out other units such as Riflemen or Grenadiers quickly, though this has limited use as Riflemen will only be grabbed one-at-a-time which negates a flamethrower's area damage.
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.
Avoid leaving the Grappler undefended, especially if it is placed near the back of the island. It will only let troops get further into the base and its blind spot will prevent it from attacking the troops it pulls in.
The Grappler performs well against the Scorcher-Tank and Scorcher-Zooka combinations - the Scorchers get pulled away, leaving the Tanks or Zookas exposed to other defenses.
It also does well against the Tank-Medic or Grenadier-Medic combo as the Tank/Grenadier gets pulled causing a few medics to go heal it only to get distracted or massacred by other defenses.
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.