You've missed the point.
Here's the problem.
Enemy is standing adjacent to the fighter. The paladin is 3 squares away (just out of reach). The paladin issues his divine challenge Mark.
If the enemy ignores the paladin and attacks the fighter, he gets -2 to hit and takes 8(!) damage.
If the enemy attempts to attack the paladin he tries to move to the paladin, the fighter gets an immediate opportunity attack (class feature) and if he hits does damage and stops the enemy moving away from him.
So the enemy either attacks the fighter and the paladins challenge blasts him to bits, or he attempts to reach the paladin and keeps getting hacked by the fighter whom he can't attack without taking damage.
Cheers