I am no expert, I did little to no research at all and i don't claim this is the best way to do it. However, on my experience this is the best way i found.

This guide intends to demonstrate an efective way to farm legendaries enchantments, mostly for the achievements.

I only did this guide because there were no others for the topic.

I belive to get legendaries you need to be at least level 30, I don't know how much the chance go up after that, but I mostly farmed on level 40 (continent 8/9), its possible that's not the ideal level to farm, I don't know.

Some enchantments need to be unlocked first:
Boomstick: kill 10 explosive espers
Triple jump: kill 5 utahraptor

Higly recommend lowering dificulty settings to minimun and having spells of your level so you one shot mobs and cartboards

Go to a grassland map
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And start entering every house/shed/etc on it.

You'll be looking for very long rooms filled with cardboard like this


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And inside of these rooms you'll finde these chests which you'll atack with a spell that affects background to get a 100% enchant container (red)

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You can get multiple chests per room, so it goes really quickly and the percentage of building that have these chest is pretty high.

Some building don't have this room/chests, so avoid them, so you'll save time. Here are some screenshot of the buildings to avoid (from the inside)

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You'll be searching for buildings like these:

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Notice that the three stores building will not have a vortex to exit the building, but a four or higher will on the yellow room.
Notice that the chests appear as a green dot on the map.

Here's a video of me using this method for about ten minutes.

It will still take a long time, and it will test your patience.

I think pretty much every kind of room except for those i said to avoid are good ones (anyone that has 3 or more floors and its not the windmill or log cabin lodge and other "huge buildings" - a good rule is if the initial floor has more than one place to go, then its no good)

Your inventory can fill up destroying the next few enchants you get before emptying it, but you can reabsorve them without going to town.

Remember that on camp you can buy head enchants which can assist on find an enchat of a specific slot, you can also buy a random enchant which resulted on a few legendaries for me.

Try not to leave this for last or you'll get sick of it really quickly, its better to intercalate between this and other stuff like missions.

It took me 91 hours total to do 100% on the game.

I've started using head enchant as soon as i had all achievements from 1 head slot.

The last one for me was the water enchant (Tsunami), since the slot has 8 possible enchants i decided to farm money to gamble instead, for that i went for really low levels of swap dungeons and overlords maps withouth killing any liutenants.