Successfully complete a chapter on the Normal difficulty setting to unlock the ability to reply any mission from that chapter on the Hard difficulty setting. Press L1 and R1 to toggles between chapters and Square to toggle between the Normal and Hard difficulty setting.
Select a good AC to start with and then get to at least Chapter 3. Start fighting the sim ACs in the ACSIS menu. A good amount of money is given for each fight, and as a bonus you get the defeated AC's emblem and schematic. You can use the defeated AC schematics to defeat more ACs because you only need a buildable AC to start a mission, and not a sim. The one drawback is that you can only use AC schematics that can be tuned to a certain level. If you have enough points to tune the AC up then you can use it, otherwise it will be highlighted in red instead of yellow or white. You can also use schematics earned in the sim online.
For an easy-to-use, well balanced AC, start with the TELLUS model when you begin Single Player mode. This AC will have a weak machine-gun in its right arm weapon slot, and a powerful laser rifle in its left arm weapon slot. When you get to the menu screen where you can select the first mission, choose the ACSIS button on the right side of the screen to enter the AC Customization Garage. Open up the Weapon menu and select the arm that has the machine-gun. Scroll through available weaponry for that arm until you locate another laser rifle like the one equipped to your left arm and equip it. With these two laser rifles equipped, you will have plenty of firepower to destroy just about anything. Taking out enemy MTs, Normals, and Nexts will be easy once you refine your piloting skills. Do the first mission, which is basically an opportunity for you to get a little combat experience and test out your AC in a real setting as opposed to the blue-toned AC Test area in your garage. Once you complete the first mission, many new parts will become available for purchase. However, your current model is still more than powerful enough to do all of the Chapter 2 missions. Just save your money and finish all the Chapter 2 missions. Do not go on the Desert Wolf mission until you have finished all the others or you will miss out on cash and possible parts. More parts will become available, as will some units of FMS Memory for customization of your AC. Go back to the ACSIS, then go to the Assembly menu and find your generator. Scroll through until you find a generator with a capacity of about 90,000 and equip it. Then, scroll through further until you reach the stabilizer section. A window will appear with two options, "Auto" and "Manual". Select "Manual", then scroll down to the "Tune" area. This will bring up the AC FMS Distributions menu. Find the section that has the area of "Generator Capacity" (or "EN Capacity"). Distribute as much of your FMS memory there as possible. If you have extra and you like to equip very heavy back weapons, add a few to your AC's load to increase the amount of weight your legs can handle before the "Weight Alert" warning appears and your mobility is greatly reduced. Save the changes to your schematics and then go to the AC Test area. You will now have Air Boost, Quick Boost, and Over Boost for much longer. This also means you have more energy to spare for your AC's laser rifles, so you can combine both quick flying and quick firing of your rifles very effectively. Another good way to make your AC even more daunting is to find a good-quality booster. Try to equip your AC with as many boosters as possible that have "efficiency" in their descriptions. The most powerful booster available has a boost cost that is highly disproportional. Look for the one that has slightly less boost power and about half its cost. This will make flying your AC even easier, and enable long-term flight. If done correctly, you can stay airborne for about thirty seconds, and Over Boost for about five seconds. To get more money, go to the Simulator menu of the ACSIS. You can take on multiple AC's one-on-one (the ones that your game displays whenever "Now Loading " appears) for money, FMS memory, and parts. They appear in packs of two to four. Not only will you get money for each individual AC you defeat, but you also get more money upon completing an entire pack of enemy AC's, and usually five FMS memory, a new part, and occasionally a new pack. From here on out, you will have plenty of money for your customizations. The best thing about the parts system is that you can buy a part from the shop and still sell it back for the exact price you bought it for. This means you can customize as much as desired, try different weapons on missions, and have absolutely no negative effects on your cash flow. The Icebreaker Defense and Water Gliding missions are especially difficult, and will probably require more than one attempt, but after finishing Chapter 3, you will receive even more parts, money, and FMS memory.