I would love to be able to practice with the enemy type of my choice to better learn its attack patterns in the training room, rather than only be able to fight fellow brothers. 

A possible way to implement this idea is to allow players to initiate a hologram or something similar of previously encountered enemies. This could even be linked to the data archive system, where you have to first unlock an enemy in the data archive(meaning it has been studied) in order to practice with its hologram in the arena. 

I feel like this feature fits the narrative of the game and the 40k universe quite well and is actually useful to both new and veteran players​. ​Please share your opinions on this, brothers. ​
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Да! А для этого нужен тестовый полигон! То о чем я и многие другие уже писали

I agree with this proposal, as I am sure Veteran Brother Donal Hageth of Watch Fortress Erioch would. 

This would be a great option, particularly for learning boss attack patterns.

basically guys you need this
https://www.nexusmods.com/warhammer40000spacemarine2/mods/233