Conan

You get this by throwing enemies into the spikes. In Level 1 you can´t get any because there are only ghosts and statues as your enemies. When you enter Level 2 look out for the wooden spikes, grab the pirates and throw them into the spikes. Sometimes the enemies will only be stunned if you throw them, just grab them again and try it. I got the achievement during the bone cleaver fight, there are masses of enemies and spikes on both sides of the arena.
Just like the other achievements all 100 have to be done in the same playthrough! Difficulty doesn´t matter, but of course it´s easiest done in easy mode :)

So if like me you finished the Bone Cleaver level with around 70-80 impalements then do not worry, continue playing as you would and make progress on your other achievements up until you make it to the 'Midnight Raid' level; the one post-elephant boss, now continue through this level and very early on you'll be at the point where there's a bunch of tents (3 of them have chests), you'll notice spikes around the area just stand opposite the first tent and wait as around 30 give or take come your way, problem now is you can't get hold of these guys they just shrug you off, however you can stun them then press B to throw them towards spikes depending on what weapon choice you're going for (I was working on my two-handed achievement) the skill/move to stun these guys are different for two-handed you most have 'Gemcutter' unlocked then press Y,X and for one-handed you most have 'Elbow Stun' unlocked then press X,Y...
there's a few more spots after destroying the 1st ship and then even more spots in the next level 'Outpost'.

start early you can start getting the kills while rescuing the crew men. Even on the first floor there are spikes u just have to look for them!!! also make people follow u back to where the spikes are so u can kill them with it. Next level is Bone Cleaver don't kill anyone until u see the spikes !! instead just have them follow you !! there are different spots with spikes but the most important spot its next to the archery gun thingy .... u have to use them twice to help the chicks ship ... again make people come to u and kill them there... there are a shit ton of enemies in this area then at the end of the level there is a spike wall... u have to get the achievement here... yes its possible i got it after a few kills and many more enemies came after i got the achievements so it is possible just look for spikes !!!!! this will save u time from having to beat levels with no spikes