The Peomen Blast: The situation almost always arises. You have Gandalf and a bunch of Knights, and he has "Peomen" (Peasant-Yeomen-combos) with some Bounty Bay gold leadership heroes.

People say it’s foolish to attack Peomen using Knights, and they are almost right. But this little attack can be used to bring a big halt to his economy by killing his Peomen and heroes.

Trample Peomen with Knights so they are temporarily knocked down and unable to attack.

Move Gandalf in and do a Wizard Blast right near the center of their mass. There is a cross on this picture to give you an example.

Do not be afraid to use Heal if things get sticky (your opponent will normally attack your Gandalf first). After his Peomens are blasted, use Istari Light and kill his Theoden, then deal with whatever is left.

Breaking Rohan's Gate: Breaking Rohan's gate using Lightning Sword was a very popular tactic in patch 1.02. Sadly in 1.03, gates were given a huge buff vs magical damage which put a quick stop to this practice.

However, at levels 8+ it is still possible to break Rohan's gate, ending the game quickly before Rohan's heroes can recharge, and saving the time of purchasing trebs.