I cannot beat the mission where I have to get to the bridge
You could play some skirmishes to level up your troops.
This is for one of the single digit chapter battles, right? If you don't want to use the shortcut suggested by Psychotext (I prefer a more drawn out battle where I eliminate most of the enemies), I recommend using mostly shocktrooper in this battle. On sniper to snipe and only lancer to eliminate the nearest tank using the watch tower. Also, on engineer to replenish ammo for your shock trooper and sniper and to repair your tank if you decide to use it. When you clear the ground area, you can advance your sniper and lancer, but keep your sniper behind walls/rubble as they are killed really easily. Also, have a scout to rush to bases and occupy them and retreat. If you must attack, only attack while crouching or using grenades. Optimally, use your action to restore health because you will be sometimes rushing into gun fire. Use your tank to eliminate other tanks.
I was going to write out the actions I did in this battle, but it was going to be very very long.