here is how to force something to spawn so you can easily get the acheivement for killing it- best used if you are trying to kill something that can only appear when fused (you will need to have at least seen the thing you want to spawn beforehand).
1. go on practice and choose 1 thing that you want to spawn
2.click on go practice then when it says click to start, right click and then choose back
3. go on nest, then hover mouse over small/big/huge nest (do not click on it, just hover mouse over it)
4. right click then choose forward
5. click on the same size nest your mouse hovered over
6. the thing you chose on practice will now spawn and you will get the acheivement for killing it
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 "The Great Siege"){kind=icon}
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 "Pyro"){kind=icon}
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 "Aether"){kind=icon}
 "Amorphous+"){kind=icon}