You can change the order of the characters in MM6, but not in MM7, TTBOMK. What may be happening is that some monsters in the game have a "Preferred Enemy", and they will attack him/her first. If he's getting a pasting, see if you can increase his armour class and apply some protective spells to him. Also, if you have somebody in your party with the expert Alchemy skill, mix some "Harden Item" potions (mix a yellow potion with a green potion) and harden your cleric's armour in order to reduce the chance of it breaking.Regarding his position, if you've got the standard "2 fighter, 2 spellcaster" party and your spellcasters are a cleric and a sorcerer, position 3 is the usual place for the cleric. The guy you want at the back is the sorcerer. He gets fewer hit points than the cleric, and he's very limited in the armour he can use. Early in the game especially, he really needs protecting.
