If you're doing Planeslayer for the first time, there are some bottlenecks and noteworthy points to be aware of. Here are some I found useful:
Raid (or related) bottlenecks:
Who: an Ungainly Beast
PH: Any of the mobs in the tunnel behind...
Where: Plane of Valor, in the tunnels behind Aerin`Dar
When: 3 days +/- 12 hour variance on Aerin`Dar
How: The area is only accessible (by clicking the orb) when Aerin`Dar is alive unless a mage was left down there to CoH
Who: Elite Knight, Necromancer and Priest of Decay
PH: Chance to spawn in the waves of mobs during Bertox encounter
Where: The side rooms/corridors downstairs (click the chair)
When: 3 days +/- 12 hour variance on Spectre of Corruption if last raid successful or after server reset; 6 hours if previous raid failed
How: Clear the waves of mob that spawn during the encounter, but do not kill all of the Kings or spawn Bertox you have all 3 Elite kills. I just pulled a King to a corner and let it beat on a toon (no damage shield or riposte). Hail the planar projection at the end if you're doing Artisan's Prize.
Who: Lossenmachar
PH: Any prismatic recluse on spider island (island with rainbow to click to Xegony)
Where: Plane of Air; spider island with rainbow to Xegony
When: ~2 hour respawn
How: Don't kill all of the spiders on the island unless you're fine with a dozen "an erratic arachnid"s assaulting you for an hour (zone-wide aggro, respawn instantly after death). They stop spawning if you kill the Avatar of Dust, otherwise you're stuck letting them beat on you. They will slowly path all the way to the entrance if you zone out/back. At 115, you can just AFK while they beat on you.
Honorable Mentions:
Who: Jopal Chieftain
PH: Anything in the Jopal Seer hut
Where: Plane of Fire; 2nd Jopal hut from the left (next to the one with the tunnel entrance); shares the spawn location with Jopal Seer
How: This guy has an incredibly low spawn rate. I had a toon sitting there for over 2 days with a pet fixed on the spawn, and it finally popped around 60 hours in. I'm guess around a 1% chance to spawn -- and yes, it really does spawn there (you'll question it after a while).
Who: Flame Overlord
PH: Flame lord fire elementals
Where: Plane of Fire; second large field with Babnoxis
How: Another rarer-than-rare spawn. Though this one is more annoying since the PHs are all over.
*Note: Plane of War is not required for this achievement.
**Note: Aside from the raid bottlenecks, the most frustrating part about Hunter achievements these days is contending with AFK MQ botters that sit on spawns for days. PoP mobs have too low HP to try and out-dps, and DBG doesn't enforce it, so you just end up waiting until they leave.