You take a bath in ketch-up, that way you're well seasoned for when the dragon eats you.
Red Dragons hit with heavy physical and heavy fire damage. Thus, high defense and high fire resistance are useful. Fire protection though equipment (esp rings!) and fire resist potions can dramatically reduce the fire damage. Ice weapons and ice attacks might prove useful as red dragons are weak against ice damage.
A summons at the start of the fight can help, even if it's just a wolf summons. Ice elementals will do more damage due to elemental resist factor but will get melted pretty quickly once the dragon attacks them. Every attack the dragon does on your summons is one less attack it does on you.
Try to get the dragon between you and your Companion. That way one of you is hitting with flanking attacks at all times. Your companion should be geared in the best equipment you get get, including fire protection.
Depending on how you use your Companion, you can make your Companion either the damage dealer or the tank that takes the hits. Between your cleric's and the ability to use healing items, you should be able to consume a lot of potions. Keep in mind your Companion can use potions, too.
Defense and attack potions should be used right at the start of the fight, and used again anytime you're at full health and can use items.
The first few red dragons you fight might prove a handful, but as you tackle them, it gets pretty easy.