You need a lot of room and patience. Try to get your opponent to agree to a 1 hour cease fire.
Prep (About an hour) Edit
The setup will take you about an hour to get it rolling, but once you have it, you can mass produce T4 units at will. The first part of the attack is setting up a turtle defense. The key to the fast build is to get your ACU to act as your primary engineer. You have to upgrade him to build up to tier 3. Basically you use him to build the first 2 land bases, 4 mass ext, and 5 power gens...build 5 T1 engineers...and focus on getting T1 defenses and increasing the land bases to T3 building. While you have the land bases upgrading, use your ACU for defense...and while you have your ACU upgrading, mass produce the tech level units the bases can for defense and annoyance attacks. The moment you get produce T3 engineers AND your ACU can produce T3 gens and mass fabs, make a string of defenses (5+ clusters) and cover everything with shield generators/missile defenses. The engineers should focus on the defenses while your ACU builds 3 T3 mass fabs for every one T3 power gen. Cover the landscape with them...think how you want to borders to expand and push your way out there. When you reach the edge of the point defense range, pop another shield gen and rebuild 5 more point defenses at the edge the new shield. What should happen is a layering effect of AAs and point defenses that can stop anything while protecting your mass fabs and power gens. During this process do 2 other things, make a gate and get 2 other SCUs to help and use 2-3 land bases to create AA units in an infinite build. T4 units are slow and easily bombed, so make mobile AA units to follow your T4's into battle. Place your AA units at the back of your base until your ready to move, don't let incoming land forces or backside hitting aircraft get through.
Building with the ACU Edit
The build rate of the units goes ACU, T3 engineers, SCUs. Depending on your needs, use SCU as the big builders...they can put up buildings/T4 units faster then anything else, and they generate thier own resources. When you think you have enough of a power/mass output, use your SCUs with at least 1 other Engineer to create your first Exp T4 Unit without any other build units doing anything. If you see a negative mass or power loss, you still need more fabs or gens. Once you are on the plus for both, get more SCUs and engies helping and watch how fast the T4s are created using this tactic. Should be able to create a 100K hp Aeon Assault Bot in about 2 mins. Less time for the other factions.
By now your enemy has been wasting resources on either attacks your defenses or trying to amass a huge army himself. The trick is, you have focused your energy into T4 units instead of anything else using the build rate of your primary ACU to make them with no mass/power loss. Wait until you have maybe 10-12 T4 units with a healthy supply of about 70 to 100 AA units to follow them. March in and destroy. After you have sent out the first wave and they are in transit, queue your ACU to build another 10...by the time you have the first wave reaching the enemy base, you have anther 10 in reserve. Normally, you can out build before you enemy can make the first Strategic missile launcher ready to launch, if he has nukes ready, spread out your forces on the march, don't let a well placed nuke stop your advance. If you hear that voice announce a launch, separate your units to keep losses at a minimum. You will overwhelm your opponents with waves of T4s faster then they can recover.