Grinding is doing an action over and over in order to raise one of your character's characteristics, or to sometimes to unlock a reward, like an item, a quest, etc.
In Legend of Grimrock, there is almost no use for grinding. The only "tasks" you can do are "Getting to the bottom of mount Grimrock" and "Killing the monsters in order to survive". Only the second task can be repeated, since the first one triggers the end of the game. But even with the second one, it's almost impossible to do it repeatedly since monsters usually don't re-spawn.
Also, since the only characteristic you can improve during the game is your experience level, there is no use at all for grinding excepted for raising your experience.
So, if you want to get more experience than you get during a normal run of the game (for example to obtain level 50 in a skill without having to put all your skill points into only one skill), you might want to grind. To do so, you will have to find a spot in which monsters re-spawn indefinitely. There aren't many of those. Then you will be able to start grinding.
Infinite monster spawn areas
The know areas in which monsters re-spawn indefinitely are the following :
level 4: Beast Gardens and Menagerie
This area is primarily aimed to be a puzzle. In order for the puzzle to be always possible to solve, monsters spawn indefinitely. The downside of this is that a pretty long cool-down time happens between spawns, and monsters can be "trapped" into the cages (after all, this is how the puzzle is solved).
level 6: The room before the Maze of madness
Here, snails spawn again and again, when you walk into the hallway between the Haunted Halls and this zone. It can be useful to farm them in order to get food, but they don't give that much experience.
level 6: Haunted Halls (recommended)
This is probably the most efficient place to grind in the whole Mount Grimrock; since you can easily trigger the spawns and the spawning monsters are pretty easy to kill (there are Undead Soldiers).
Here is a map of it:
Legend :
Each colored tile is part of the monster-spawning mechanism. Tiles of the same color are part of the same "spawner", so there are three spawn mechanisms in total. For each one, an Undead Soldier will spawn at the cross if you stand on the tile with the thunderbolt symbol. Note that there is a short cooldown period between each spawns of the same spawner, so you might want to walk around triggering all spawners in a row (and then start again).
level 6: Fireball Hallway
In this room, one Giant Crab spawns regularly. But it takes some time to kill, even if it gives an important amount of experience. And the delay between the spawns is pretty long.
level 6: Halls Of Fire
Three Uggardians spawn here when you fall into a pit and come back or just wait. They give a fairly important amount of experience (500 XP), but they take some time to get rid of. Plus, having to fall into a pit and come back takes time. The third Uggardian spawns when u step on the plate near the torch you removed.
Level 8: Tunnel Ogre Pit
This area is accessible by falling into the pits in northern Level 7. One Giant Spider occasionally respawns in this area.
level 9: Temple Grounds
In this area, Ice Lizards continually spawn. But again, they don't spawn that often, and sometimes, only one of those spawns. They give 675 XP per kill and always drop Ice Lizard Steak.
Tips for maximizing experience
Experience per second rather than experience per monster
While grinding, keep in mind that the goal is not to kill the monsters which give the more experience, but to kill the monsters that give some fair amount of experience AND die pretty fast.
Experience bonus item
You would better equip the Spirit Mirror Pendant for a 25% experience bonus while grinding.