You need a vehicle, any vehicle. Get that and a technician and drive the vehicle to the back of the base. There is a ramp there that you're not really supposed to be able to pass with anything but infantry, but you can drive up and over it. Proceed forward until you reach the GDI barricade. There is a bunker there, drive behind it before the AGT destroys you (plenty of time). Get out of the vehicle and leave it there. From there you can walk right into the refinery without getting shot at from the AGT. When done, go back to your vehicle, drive back to base. You can't get back into your base so blow your vehicle up (to prevent vehicle limit from being reached too fast) and then you're done.

This tactic is very useful early on in the game if one person buys a buggy and one a technician, particularly because noone ever notices the buggy and even if they do they think the barricade will stop it (unless they know about this tactic).