Haven tactics.

Humans are primarily a defensive race; you should develop their defense as much as possible, as gnomes have a weak attack. You can take "Readiness" (griffins become even stronger after that). And to surprise them... if a Knight casts Armageddon (it's funny), and you also have a Fire Lord, it will significantly reduce the gnome's defense... then the gnome will be very surprised)). But seriously, I agree with Major – there's not much to surprise them with.