Easy kills
On the first level, get three squads of riflemen. Keep them in the trench. Get another rifle squad. The men will continue past the trench. Get the other three rifle squads and move them out of the trench to follow the fourth squad.
When attacking, first fill up your bunker with assault troops. When it is full, send out another squad. As soon as they get in the bunker and start to get back out, send out all of you men in the bunker out with them.
When the enemy comes out of their trench, fill your trench with riflemen. By doing this, you can easily kill them if they approach.
Fill your trenches with machine gunners. Keep on sending out assault teams. Because the assault teams throw grenades, you are getting free fire support.
To get rid of snipers send out riflemen.
If you have a tank, always send it out first.
When fighting at medium range, use snipers combined with an assault team. The snipers have a long range and will drop back while the assault team advances.
When fighting at long range use machine gunners with an assault team or riflemen. The machine gunners will be covered by the riflemen or assault team.
Whenever charging, fill up the nearest trench then send out one more team. As the team gets in they will automatically jump back out. Time it so that you can send out the rest of the trench with them. This may result in bonus experience because of the number of squads you sent out.
When playing in Skirmish mode without artillery, put one machine gun unit in a trench, followed by two rifle units or one rifle unit and one sniper unit.
Get the first trench filled with Lewis machine guns. When fire support kills some of them, send them all out. You also can do this with assault teams for better results.
When you do not have artillery, use lots of guns to decimate your enemies' morale. When you have artillery, capture the ground quickly to avoid being blasted to bits. When trying to decimate your enemies' morale, fill your trench and keep them there. Occasionally making a charge will cause your enemies to reach you faster, thus quickening the game. The best defensive combination is two machine gunners and one sniper. If you have a captain or a tank, send it out first. Make sure your captain is at the very last trench on your side. When you have occupied a few trenches, keep one or two defensive squads in each. The others can be filled with riflemen and assaulters. The best way to gain a trench is to knock it out with gas first. If you do not have gas, wait for the enemy to make a charge. Wait for them with a full trench. When the enemy is reinforcing, order the charge. When you get a tank, it means that your opponent has one too. Send out a few assault teams first, then the main riflemen.
Usually during a campaign there are two trenches. To capture it, you can subdue it with firepower first. However when there is no support, keep your own trench full, then recruit one more squad and make the charge. Usually having a combination of differing troops is best. When in a trench you have unlocked poison gas, do not keep too many there. One shot of the gas will send your morale plummeting. Also when sending out tanks, they will barge through to the end of the battlefield, so you cannot control them. Bringing them onto the field adds morale, but their destruction destroys morale.
When your enemy sends any squad, send riflemen into a trench filled with one Lewis gunner and two assault teams. When the enemy approaches, send the riflemen. You will now have four teams. You can do this again and get five, six, seven, or eight teams if done correctly. Assault teams are best for taking out trenches. Snipers and sharpshooters are also useful.
Go to "Custom Games" and select five trenches (the maximum) and two barbed wires. Set the map on the longest possible. Activate everything except for tanks. Do not have artillery or bombs. Get some machine gunners and send them three trenches up. Wait for another machine gunner then do the same thing with them to bring them to the third trench and another one so it is full. Put riflemen in the trench back from it. On your closest trench, put full machine gunners. Get a grenadier and charge them with the riflemen.