It should go more like this: Paladin issues his Divine Challenge, he and the monster trade blows for a round or two until they've both hit eachother. The monster now has a vested interest in finishing off the Paladin, because if he doesn't, the damage he's done to the pally, and all his actions up to this point have been 'wasted.'
Except of course, a) Probably missed the paladin so no damage done, or b) paladin got healed, so no damage done, or c) monster has an INT higher than 3 and realizes a+b will happen so never even goes after the paladin in the first place.
The solution is for DMs to play monsters like they're dumber than dirt, otherwise no monster in their right mind would engage the party let alone try to kill the hardest to hit and kill person in the party first.
Despite the kludgey and entirely irksome death-beams-with-no-save that paladins throw off, 4E has not mechanically solved the problem of making tanks, sorry, "defenders", sticky.