Eradication (Red) mages are powerful on offense, but their defensive abilities are often relatively weak. Red dragons, in relative large numbers, are a good second stack, but they are costly and you will notice their weakness against cold attacks fairly quickly. Most other eradication units simply die in droves, which bodes ill for eradication mages defending against land grabs.
For that reason, and that the Red Dragons don't use too much mana, try some liches as supplements to your army. The current relative weakness of eradication creatures almost necessitates that eradication mages seek non-eradication units. Eradication mages also have the benefit of using spells such as Gravity Pull, a surprisingly effective strategy (many opponents, particularly ascendant mages, seem to favor flying creatures or have no choice).
The next best eradication unit, however, is perhaps the hydra. They regenerate, and when used with items that promote this ability, hydra can be successfully incorporated into an eradication mages army. The primary problem faced by eradication mages is money production. To generate a significant income, you must have lots of people. Eradication mages need lots of farms and towns. Eradication mages often run at negative in this resource, and any eradication mage who wants to be highly ranked must learn to run at a high negative in income. Beware using units that consume your population, such as Lich and Vampires or you may all too quickly find yourself with a problem you cannot easily solve. Perhaps the best defensive spell and item combination for eradication mages is Inferno and Strange Metallic Can. Inferno costs lost of magic power, but it usually wipes out your opponent's weenie creatures. Since land loss is proportional to the number of surviving units of the attacker, weenie units are used to maximize land taken. And since Inferno wipes out most or all weenie units, it really minimizes land loss. The Strange Metallic Can has been known to bring back red dragons, which never hurts. This combo minimizes land, creature, and power loss.
To maximize land gains however try using a weenie troop of your own. Zombies are the way to go here with a side option of Nymphs for those who like to keep the beauty levels up.
Finally to maximize gold production get yourself Alchemists and Merchants. They will help you run with even more Dragons.
A simple but in lower ranks effective strategy is using Lizardmen only and the spell Flameshiled, which will give a random stack (i.e. the Lizardmen) Fireburst ability. Because you tend to have many Lizmen, it'll be a mighty fireburst. With this strategy you'll win every battle, as long as your opponent uses Zombies, Skels, Doms, Archangels, Angels, Knights, Knight Templar, Ghouls or Treants, as most people will.
This defense is easily broken by skilled players, so use it only in low ranks. It's an interesting opening to keep your heads low at rank 1.500 or so and gather about 4.500 acres with this strategy, which will not take long, before finally playing seriously for top positions.
Also, because of your defensive weakness, high forts and barriers are a must for every red mage. 2% - 2.5% of both are suggested.