The achievement requires you to kill 15
DIFFERENT ancient mobs out of a possible 21 variants. So you'll have to kill the majority of them.
Ancient mobs appear randomly in the Ancient hunts and require specific offerings before they will spawn. These are made by offering equipment pieces so you're going to need either a lot of them or very specific ones.
Each piece of equipment is assigned a rune (or two) and you will need to offer x amount of different runes for the chance at them spawning. The chances of them then going on to actually spawning are shown below the offering and can be increased by offering up those precious Enchantment points shown in percentage. If you don't offer the correct runes, they won't appear in the first place!
I used the Minecraft wiki for the list of mobs and the required runes.
https://minecraft.gamepedia.com/Minecraft_Dungeons:Enchanted...So, my tips are as follows;
- Initially try and get gear with multiple runes offered to get as many different ancients to spawn as possible. A black box with a question mark means you haven't beaten that ancient before as it is unknown.
- Difficulty doesn't affect their spawning so you'll only be making it harder on yourselves.
- Only start targeted runs with a single ancient when you've beaten the majority of them and struggling to get multiple to spawn from runes.
- I personally avoided having attempts with already defeated ancients as it had the potential to spawn those again rather than ones that I had not already killed.
- If you've got the enchantment points to spare then use them! They help a lot!
- Once you've killed an ancient you can quit to the camp and keep your credit towards the kill. No need to finish the map.
- Use the map all the time, explore every nook and cranny for those caves! Learn how it works also, the glowing caves needed don't spawn facing away from the camera so you can avoid going down certain paths and save precious time.
- Some maps will have up to around 3 ancient spawns max, However, they may be multiples of the same one. Though there is also a chance you will get nothing to spawn.
- Ancients spawn in their own unique area, so after going through a cave and if you appear in the long tunnel the only thing at the end is a few chests. You'll know it's their area by the change in the biome or the area will generally be a big arena circular shape.
- Certain biomes that you appear in will have no portals in them at all (from my experience at least). You can learn which ones you can pretty much skip.
- Make a list of the mobs and tick off what you've done to keep track, it helps!
- The achievement tracker goes up in stages so won't track exactly, it just roughly every 3 different kills.
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This will likely be your last achievement of the DLC so you can work on the other ancient hunt related ones at the same time. I always had a bat, wolf and a soul entity on me at all times whilst running through and looking for their related areas for the "Homecoming" achievement;

The Homecoming achievement in Minecraft Dungeons worth 131 pointsDuring Ancient Hunts, bring a wolf, a bat and a soul entity to visit familiar locations.You'll need to look in the spider cave biome, woods biome and the caved nether biome (not the red one, the brown soul sand area).