Killing Mages
Mages are very expensive, the best way to attack Mages is to limit the enemy's ability to mine Gold. If that is not possible, attempt to attack them directly using Bloodlusted Ogres. Cast Runes around the Mages and in front of them in the hope that they will walk over the Runes and be destroyed.

Invisible Mage attacks
There are only a few possible defenses against Invisible Mage Blizzards on the Peons. You can place Runes around the area, but the player will most likely walk around them or wait until they disappear. You can place Towers or Cannon Towers around the Mine so that once the Mage starts casting Blizzard he will be killed before his spell causes serious damage. But quite often Mages can find a perfect place to cast that is out of range of the Towers. Another option is to place a few Catapults around the Gold mine that will instantly kill the Mage when it appears. This also is usually quite impractical. The best solution is to wall in the town completely using buildings and walls. Invisible Mages cannot walk through buildings so buildings are the best bet at protecting against such a strategy.

Mage/Paladin Healing combo
Orc players often head straight for the Mages. Humans may Slow incoming Bloodlusted Ogres and cast a Blizzard on them then run their Mages toward your Guard and Cannon Towers. At the same time the Paladins beat on the Ogres backs, and also Heal the Mages as they are being attacked until the Ogres die to the Paladins and possibly Towers. For the Orc player it is best to attack the Paladins first as they are the ones Healing the Mages. After the Paladins are gone, the Mages will be without protection and more easily killed.

Try a surprise attack. If possible, use Demolition Squads to blast your way or Transports to land your way into the fight from an unexpected direction. This can catch the enemy off-guard and unprepared to defend against your assault.

Exorcism on Death Knights
Try if possible to get rid of that Church before they get Exorcism. But if they do get it and use it correctly, you might avoid making any further Death Knights. Unholy Armor can prevent Exorcism if you need to make a quick hit on the enemy town. This will require more than two Death Knights.

Fighting Invisible Transports
Invisible Transports usually can't be Invisible all of the time. Often people with Invisible Transports bring them right back to their island when they are done attacking so watch their shores. Wait for the Transport to appear off shore or land back on the enemy shore then sink it using Destroyers.

When you see units unloading onto the shore quickly cast Death and Decay where you see them unloading, use Ogres to place Runes around the the Transport. Build Towers out of range of ships yet still within range to attack units landing on the shore. Often the enemy will not have time to unload before you can destroy them using a combination of Towers, Runes and Death and Decay.