1) diversionary assault:
Get a few, or one or however many guys u see fit to send, guys to run into the enemy tunnel. then get all your friendly tanks etc, to pull and all out frontal assault on the ATG/Obelisk. When the ATG/Obelisk is busy beating up your frontal force, sneak in the back door with beacons and c4, blow up base power first and then proceeed to cause as much destruction as you wish to.
2) trojan horse
A) steal an enemy vehicle, start a full out assault on enemy base and run in with the vehicle. i suggest and APC or buggy. people won't notice (though the ATG/Obelisk will) pull around behind the power plant in nod or weapons factory in gdi. hop out and kill.
B) use the harvester as a sheild (someone else mentioned this in a different post) follow it in (works best while cloaked) keeping it between u and the ATG/Obelisk then plant c4 in the tib refinery harvester garage.
C) this i ahvent tried but it's a good idea, let me know if it works or not. put c4 on an enemy vehicle retreating, or on your own. set off c4 when vehicle is next to building. try the harvester whne its droppin off its tiberium. dont know if that'll work but its worth a try.