If a factory is weak to something, such as Jumpbots are to porc, but they have a unit that counter that weakness, spam that unit and your opponent will be caught off guard and go down easy if they are playing to the meta.
I really don't like when the game runs long so I compiled a list of known rushes.
Rovers: Fencers; Scorcher, Ravager
Jumpbots: Jack, Pyro
Shieldbots: Rogue, Snitch; Bandit, Thug
Gunship: Blastwing; Gnat, Locust
Hovercraft: Dagger; Mace, Scalpel
Heavy Tanks: Ogre
Cloakbot: Glaive, Imp; Scythe; Reaver, Glaive