Defense... in all matches I find, people mostly go on full offense when winning, or full defense on losing. The guys I find in base were usually the ones died in the field. Mines are good, but if you got no one or only one guy posted inside, it's almost likely that the base will fall
I find that as GDI, guarding your base in Field becomes less tedious if you have an APC posted on the back side of the base that is not covered by the AGT. Couple of Nod sneakers actually thought twice before exiting when they saw my APC right on their face, and instead of planting their nuke on the ref, or bar, they planted it inside the cave, which... either got disarmed or failed to hit the target
In Island, posting a Humvee (never tested Buggy) or APC on the tunnel entrance usually works well. They actually works like a machine gun turret ready to mow those people who gets past your mines
Mammoth and meds are actually not a good idea though, unless maybe you're forming a barricade against APC rush. I managed to shoot down a nuker inside a chinook with a mammy btw. And those heavy armor might need some explosives to chip down
I haven't found a viable tactics for Nod, but I think being a flame thrower works well to counter infantries in APC rush. We saved OB in Gold Rush and I soaked the entrance of the OB with flame