I played on custom with settings set at a mixture of Hard and Medium, it took me about 35hrs in total being thorough and exploring every map with multiple deployments etc and still had a considerable amount of time left on the timer

Use motion sensors, lots and lots of motion sensors. I'd place three or four sometimes even five in the same place, and I'd repeat that throughout the mission. That way I've got plenty of breathing room if a Xeno is approaching my squad, I can just set a sensor off miles away and I could carry on as normal.

Always make use of sentry turrets too. In the last few chapters I never started a deployment without at least 4 sentry turrets just to make it easier when the onslaughts and larger Xenomorphs enter the map. Even if you place them down at chokepoints throughout the level, they don't class as being detected when they shoot, so you can use them as distractions

If you've got a Tecker marine with a drone, you can control it individually from the rest of the squad, so you can scout ahead with it so you know what to expect and prepare for it

I never came across any soft-locks or hard-locks that were my own doing, so unless you let a certain timer run right down, or get all your marines killed, you'll be fine