So a white room with an elite standing still behind a counter with all your Raiders weapons laid out in front of you so you can take cover and line up your headshots on a stationary opponent? That won't help you get any better at this game. If you could be a little more descriptive about the controlled environment I might be able to understand where you are coming from a little better.
Think about it like Warframe. You have an arena or room (maybe even have some control over the layout of the room) where you can spawn in (single or multiple) enemy types (war dog grunts, Hades commanders, etc..). Once spawned in the enemies can be set to passive (non moving targets that don't attack, basically shooting dummies) or hostile (aggressive, actually attack and have access to all their abilities). Since enemy responsiveness, damage and health are determined by MMR there should be some sort of interface that determines the effectiveness of the enemy AI. There should be an option to quickly switch between weapons (an added bonus is that you would be able to determine the forge rank of the weapon in the training room), cards and characters. Also have a way to add aleph to your character and the AI to test out effective counters against enemy abilities and how effective are cards when boosted. No timers, no objectives to worry about.
Good enough of an example ?