First, my favorite breaching formation in Door Kickers is having an operator approached near the door from the side and another operator aiming at it.


This can be changed too, depends on the situations, map's layout and number of avaiable operators, you can add more Assault operators to aim at the door and send a shotgunner to shoot the doorknob etc. The good thing from using this formation is that you can shoot enemies on the other side of the door faster than having SWAT units kick the door and rush in at the same time. They will have much slower time to ready their weapons and have more chance to get shot from unexpected locations.

This formation is suitable for slower paced playstyle with a greater chance to keep all of the operators alive.

This formation is useful in some other tactical shooting games too. Like in Counter Strike Global Offensive, the map de_Nuke has a openable door near the bombsite. If you can work with your team and ask a teammate to open the door from the side while you are aiming at the door, it will be much safer than opening the door by yourself. Or in Insurgency, a hardcore tactical shooting game, this formation can be changed for securing cornors of buildings or walls. Having teammates to watch cornors to the right and left while you are aiming at the center will reduce a lot of risks when fighting againt enemies, it's always good to have a friend helping you out.

This formation is being used officially in combat, I believed. Because the breachers have more chance of winning firefights when they have more time to ready their weapons. (This is why SWAT units usually keep their guns ready all the time when they enter buildings) It affects the perspective and easiness to be targeted of operator's body too, the further away they are from the enemies the safer they will be. Just like in other shooting games, bigger targets are easier to shoot than the smaller ones and Door Kickers did this logic very well. Enemy bots in Door Kickers have lower accuracy when they have to shoot far away targets so keeping distances is a good idea ,but you may don't want to use this strategy if your operators are equipped with CQB shotguns or bad-accuracy SMGS.

Breaching is all about clearing just one building or a few rooms, so plan your moves carefully, just a little mistake can cause you to lose a teammate due to the risks to get shot and complex building layout. Take time to think and find oppotunities then develope tight tactics and safe moves toward the objectives.
Gatering information is important too, if you had played Tom Clancy's Rainbow Six game, you might know how important the information about enemies and hostages location is.
Using a spycam to see under doors into the other room is a very effective way to gather information for your squad's plan. Running blind into a room full of bad people carrying AK47's is sure not a good idea.

But, if you want to be better at these kind of games, just don't rush and go YOLO because this will give you nothing but failure...
even if you succeed, it's not from your intelligence.

In conclusion, tactical games are difficult and require a lot of patience to play and learn. It just matters if you can handle or not.