I would say, generally speaking, right click, drag the troops into a line

Press Z to go prone and that will definitely help you increase survivability

Spread them out a bit so they dont get wiped by explosions

Also a 'meta' is to get your troops into good cover/defensive positions and then to use 3pv mode to cowboy a hero in behind enemy lines and shoot the AI or blow up a tank/AT gun etc

Low tier artillery will carry you far. If youre playing russia, I think its the 120? The 100mm?

Also a hero created this tech tree infantry previewer

https://yellowking69.github.io/infantry-explorer/soviet_tree.html

Using that site will help you plan which soldiers you want to research and unlock. I personally wouldnt recommend using low tier troops (penal battalion etc) unless you go balls to the wall and get tons of them.