The easiest place I found to get this was on "Underworld: City of the Dead".
When you first enter this location down in the bottom right of the map you will encounter a couple of flowers then after jumping up a small ledge a spider will appear. Jump over the spider and you will find some spikes that can be raised and lowered depending on your spirit form. The first spikes are red so make sure you are also red so that they are below the floor. Wait for the spider to walk over them then switch to blue to kill it.
Alternatively, you can also get it a little earlier in the game as the first part of this video shows: