Crypt Kicker

There is a bell @72.4, 49.4 with a venthyr standing next to it. At the middle of every hour (X:30), the bell will spawn with a friendly ghost, which can be repaired for 30 Infused Rubies to get a buff. If the ghost is Gahiji the Tomb Raider, repairing it will give you a 20 minute buff that increases the chance to get keys.

It's a lie; the buff rate is not noticeably higher. Instead, certain enemies nearby will have a buff that forces them to drop a key. My first bell I got 10 keys, and the second I got 7. If you can server hop, the same bell will be up on the other server (unless it's been repaired) and the same mobs (in my experience) will have the key buff.

If you park your character in the water next to the bell and check back every hour til the bell is up, you should be able to get your 50 keys in less than an hour played.

If you are an achievment hunter (like me), do yourself a favor and grind this as a necrolord.

Yes.

A necrolord. The keys will drop for any covenant, and you'll want to open as many as you can for the Small Posable Skeleton item to drop. (Wadrobe Makeover and Fashion Abomination achiev).

I already had this achiev and now have to go back to grind keys for the abomination factory item :/

I farmed [item=173735] very easy.
Step 1: Go /way 69.41, 45,75
Step 2: Kill [npc=158892]
Step 3: Server hop (you can jump in the quest groups)
Step 4: Repeat.

The respawn is very fast so you'll get the achievement.