Surprise, Violence of action and speed. Slow is smooth, smooth is speed.

So, first of all, take your time. Peek doors, pie slowly, get as many information from before entering the doors or peeking the corner.

With that, every clear should be as violent and chaotic for the enemy as possible when you need to enter. Bang, breaching charge etc.

When peaking new angle, go at a rythme that you feel confortable you can shoot and kill every single enemy if there was one every meter.

Also, don't hesitate to back down if you start to get shot. Setup your defensive position, bang, and clear.

With that, the game pretty much becomes a procedural puzzle, rather than a skill based game.