Put 6–9 rebels on Intel in actively contacted regions. This generates missions with long expiration timers, giving your squads time to infiltrate properly.
Long War is less about running and gunning and more about slowly solving a tactical puzzle. You cannot afford to take fair fights. Always try to: xcom 2 long war guide
Here are some overarching strategies broken down by campaign phase. Put 6–9 rebels on Intel in actively contacted regions