Note: This procedure requires editing a game file; creating a backup copy of the file before proceeding is recommended. With a text editor, edit one of the "level_*" files in the game folder. Locate the "//Tank, Jet, HelicopterGun, HelicopterRocket" section and decrease the four numbers following the "Aggression" entry to lower how difficult the enemy tanks, jets, helicopter guns, and helicopter rockets (in that order) will be in that level. Note: The numbers must be from 1 through 9.
When the boats that carry the troops and tanks appear, don't wait until they hit the beach and the gates drop. Instead of trying to gun down each soldier that comes out of those boats, use the howitzer cannon and try to destroy as many of the boats as you can before they hit the beach. Do this quickly and accurately while trying to hit each boat with the first shot. You will not be able to get all of them before they hit the beach unless done quickly. For the remaining boats, use your machine gun and gun the troops down as they appear from the boats. The troops will be in more of a group formation. Only use your missiles on the jets and helicopters. If you run out of missiles and need to shoot a jet down, use your machine gun and wait until they dive towards you. Shoot away at them to take them down.
Note: This procedure requires editing a game file; creating a backup copy of the file before proceeding is recommended. With a text editor, edit one of the "level_*" files in the game folder. Locate the "//Bullets, Projectiles, Missiles" section and increase the three numbers that follow the "Ammo" entry that is underneath to set the amount of bullets, projectiles, and missiles (in that order) that will be available in that level.
Another change that you can make is the artillery. The higher you make that number, the more mortars will be launched when you press A.