This is the only location to farm Gruesome Ribcages. Start at Grendich Courthouse and travel through Diessa Lowlands and Flame Temple Corridor. The foes along the way are L10 or less, so you should be able to simply run past everything without fighting them. Circle the southeast quadrant of this area (avoiding the non-Abomination foes), then rezone at Flame Temple Corridor. There are roughly 20 targets; expect around 5–10 ribcages per run.
The biggest threat on this farm is time: bring a speed boost to counter the Tar, the long distance to the farm, and to get away from mobs along the route. The second biggest threat is getting mobbed, so keep moving.
The Abominations mostly wander around in pairs, so use a bow to pull them towards you.
Bring a speed boost, at least one heavy spike, and at least one self-heal. Area damage is not very useful because the foes attack at range without bunching up. The abominations will create low level minions from the corpses of their colleagues; you can ignore them because they are such low level (or: exploit the corpses yourself or pull the abominations away).