Rescue Operation

I just finished this achievement today and here's what i have to share:

Unlike the horde counterpart, Warspear Prison keys appear to only drop from Warspear Blood Guards and Warspear Centurions at a low drop rate (about 10%).

Warspear blood guards are found in pairs along with a raptor rider along path of glory in areas controlled by the horde. For instance, if the stage of battle is at Crossroads, these npcs will be found along volrath's advance, emberfall tower. They and raptor riders are susceptible to any cc effects, but have a short leash range. They do not reset hp back to full when they leash, to a solo-er's advantage.

Warspear Centurions are almost impossible to solo and the only means to effectively farm them is only when your faction is dominating and is at the stage of High Warlrod Volrath battle. The npc is located right at the entrance across the bridge and will be pulled and usually killed so that your faction can advance towards the boss. Just tag, help to kill it and hope it drops the key.

To access the prison building without going through the front entrance and likely to get killed by horde, there is an alternative entrance into warspear stronghold north of Ampitheater leading into the stronghold, this is the same route which horde enters ashran from their capital city, you will exit ashran briefly but not long enough that you lose your reserve spot. Using this route ensures that you are less likely to be seen but you want to use stealth/invisbility effects to stay out of sight of nearby horde.

For the npcs you have to rescue, only Marshal Andrea DeSousa is in somewhat plain sight, but opening the cage from the other side will not cause the nearby npcs to aggro onto you. The cast time to open cage is 5s - haste %. The other 2 npcs are in secluded underground of the building where a prison guard guards them. She has 585k hp and spams a range attack that hits for 20k which can be somewhat challenging for a class to kill solo, after she is dealt with you can safetly free the other 2 prisoners.

If you have a paladin friend with [item=112005] you can duel him in Ashran and he can send you to the opposing faction's jail. This is much safer than trying to sneak in yourself.

Update: finally, the Warspear Prison Key also drops from the Prison Guard in the Horde Base. It's an undocumented patch note, but I can confirm it drops now.

The Prison Guard has 600k health and doesn't hit as hard as the Blood Guards or the Centurions, so it's much easier farmed. It is also in a secluded area where no Horde comes and no NPC's come apart from the Prison Guard, making it an ideal farm spot. Respawn time is between 5 and 10 minutes, so make sure you don't go auto AFK.