This is the forum for helping you out if you're stuck in Grimrock 2 or if you want to discuss about the mysteries of the Isle of Nex. Warning: forum contains spoilers!
Just lure them to the edge and jump onto them
Beware! Doesn't work if jumping off an edge with a ladder leading down
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Yes it does. What you're noticing is that monsters try to move out of the way when you jump on them. If you jump on them while they're already moving or turning, they won't be able to move away and will be crushed. Ladders have nothing to do with it.
Actually you can kill them at a ladder, as the person before me as stated. Also, if you use the Zarchton Harpoon weapon you can attack underwater and kill them.
There is three ways to get this weapon, one is from beating the guys in the water by falling on them and the other two are from treasure chests. Another is dug up in the west side of Sleet Island and the third is in either Ruins of Desarune or the Archives (don't remember exactly).
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There's another way, but I doubt it's any faster then falling on top of the enemies.
If you get annoyed with waiting for the enemy to move into the perfect spot, you can also throw items at them while underwater. To be clear, I mean picking up an item and throwing it as if you're throwing it away, not equip a throwing weapon and use it.
Yes, it only does a few damage each throw, but there's no cooldown on it so you can repeat it for about 5 or 6 seconds, and then go back up a ladder to get a breath and repeat.