The best way is flyers, moving around your empire and patroling. The spell Call of the Winds is excellent. I usually use this spell offensively to attack an enemy for the surprise value. Then any surviving flying leaders gather the remaining hawks and patrol my empire from then on. I usually have 10-20 hawks under 1 hawk leader, excellent use of 5A gems!
Also another cheap way is Ring of Warning +10 patrol bonus on a cavalry leader.
Failing that a big army under a leader moving around, but this ties up alot of troops. Light cavalry is excellent at this job, horse brothers are very good if you get them.
Of course if your facing the +30 stealth bards of Man or illusionists, +30 stealth/glamour then forget it you just have no chance.
Finally if you ever by chance do find an illusionist owned by me, duck, cause a thunder strike will be coming your way, followed by several lightning bolts...
