If you want all your grunts to attack 1 wall instead of multiple ones, you can place one grunt to target a wall, and then drop the rest of your grunt, and then they should all attack the same wall.
Grunts are weak when breaking through walls and a group will tend to target multiple walls (if you do not follow the strategy above) so it is advisable to use Siege Bombs to break through.
Players sometimes spread Grunts to avoid them all being picked off in one group by a Mortar or a Watch Tower as these Defensive Buildings deal splash damage.
Large hordes of Grunts are very effective as they will quickly destroy anything in their path. This is balanced by the fact that the Bandits are not very effective under heavy fire from splash damaging buildings.
The Grunt has no preferred target when attacking; he will simply attack the closest building to him.
They are good to take out lone buildings, like Worker's house placed far away to prevent full destruction.
If a Mortar is exposed and otherwise undefended, a single Bandit can easily destroy it as Mortars cannot target troops which are close to them.
Defensive Strategy
Grunts are an excellent choice for Clan Castle troops as they have relatively high health and damage for single housing space troops. They are capable of soaking up a significant amount of damage, delaying attackers and allowing your fixed defenses take care of the rest. Because their housing space is only 1, you will be able to have a large amount of them in your Clan Castle, making them more time consuming to lure out.
Trivia
Of the three Tier 1 troops (Grunt, Musketeer, Pillager) the Grunt has the highest health and does the second-most damage (Pillagers of similar level have higher damage per second). He is also the slowest-moving but fastest to train.
Along with the Pillager, the Grunt is the cheapest Grain-based troop in the game.
Training Cost