Ancient Kappa Shells are only known to drop from the kappa mobs in the Undercity. They are water elementalists and use the stance Turtle Shell which significantly reduces the damage they sustain from non-critical hits. In Hard Mode, this stance reduces all damage received by 26, prior to armor calculations. As such it is important to bring either massive criticals or stance removal (preferably both).
A reliable solo farm build is the perma-shadow form assassin using dagger attacks. A perma-sliver assassin is not recommended, as sliver armor will generally not deal enough damage to overcome the Turtle Shell. (Note: This statement is disputed for normal mode, but may be true for hard mode - see talk).
A perma-form cryer is very effective at taking out all aggroed mobs in one blow. Just ball up and degen the mobs to death. 3 minutes run average.
A 600/Smite monk or ritualist also can be used. In this case, the ritualist variant is somewhat better, as the life stealing from Vengful Was Khanhei is unaffected by the damage mitigation. Mantra of resolve gets removed and somehow they remove Spellbreaker and other enchantments. Once mantra is removed they can also interrupt and knockdown.
Stone Grawl necklaces are only dropped by Grawl in the Charr homelands areas. The biggest concentration of grawl in any of these areas is the eastern/south eastern part of Sacnoth Valley.
A sliver perma assassin can easily farm necklaces in hard mode. Just run to Rragars. Outside of Rragars, there are large mobs of grawls and stones. Aggro them all and bring to the 2nd tree to the left. Go to the stuck spot. Now DC to the outside and they are now stuck. Sliver them down and rezone. Takes about 2 minutes.
A 600/smite duo is also fairly effective, but again the healers can make this a slow process. (try bringing Sympathetic & Ancestor's Visage and attacking Priests in melee, energy drain should keep them from healing and take out group in one cycle HM)
With patience a spirit spamming ritualist can kill every last grawl in Sacnoth valley in normal mode. This seems to net about 25 or so necklaces on average. Be mindful of the two large grawl boss groups, as it can be difficult to properly pull small groups.
Another helpful idea if farming with a friend of team is to grab the Siege Devourer outside Doomlore Shrine on the way to Sacnoth valley. The devourer's siege attack can help to counteract the powerful healers.
Team farming builds should consider bringing a character using a variety of protection prayers skills like Protective Spirit and Aegis, as most of the grawl will be using high damage hammer attacks. A monk bonder with Life Barrier and Life Bond can be quite effective at minimizing the damage in both normal and hard modes. A spirit ritualist with communing spirits, such as Shelter and Union, can also serve this purpose effectively.
The Deldrimor Bowl provides all the entertainment, and manes, one could hope for. Nicholas can be found on the Western side of the Bowl, by the (2) Dwarven collectors and the material craftsman (Rothgar Hoofsplitter, Crafter Torgil, and Lenn Giantsbane). A single run through the bowl provides about 7-10 manes, and a number of (insignificant) armor pieces and weapons. Unless you have the quest To Kryta: Refugees active, circle the bowl from the right side, leaving from Beacon's Perch since most of the Centaurs are in the Eastern and Northern sections of the bowl. You can use pretty much anything, Triplechop/100B, Incendiary Arrows/Barrage, Spirit Spam, AoE Nukes, it all works well. If you're looking to do it in HM try a 55 SV/IP Necromancer. Or you can Vanquish with a 600/Smite train, but bring Spirit Shackles for the Protectors!