Giant Rock Crabs in the chaos tunnels.
From the 5 entrances in wilderness, this is the middle one.
Number 2 entrance in the picture!
Info about Rock Crabs:
http://runescape.wikia.com/wiki/Giant_rock_crab
They are lvl 137, they have very high defence but they are very weak to magic, thus making casting preferable way to kill these. They drop 3 charms at once, all the same colour. They drop all colours but 90% of time gold. The drop rate is ~70%.
Tanking with Salamanders
I personally use Salamanders (Swamp lizard, Orange salamander, Red salamander, Black salamander) and wear dharoks armour because salamanders ignore negative magic attack bonus.
It is recommended to have 70+ defence and wear some Barrows armour and take lots of food with you.
Ancient magicks
Alternatively, you can kill these using Ice Burst or Ice Blitz if you are a mage pure and don't have a high defence level. This method can be quite tricky though, because there aren't any 100% safe spots and it's a multiway combat area, so you can get attacked by many crabs. If you have prayer 43+, you can pray "Protect from Melee" and drink prayer potions but this is very expensive.
EDIT: I got an idea: Many of these monsters can go to 1 tile, so I'm thinking if it could work if you pulled all of the monsters in the room around you and then nuke with the ancient magicks area spells? I will level up some combat skills and do desert treasure, then I try this and check how fast it is...
Enjoy your _charmz_