Q:

Is Reaper's Challenge really that broken? Did I get it wrong?

I'm DMing a 4e campaign and the PCs are level 19, now, but I have been having trouble with a power. Reaper's Challenge.
My group's swordmage saves it through every adventure so he can use it on a Boss (solo or elite monster) effectively making it the easiest combat for the group. And worse, sometimes, when he uses this power, the group's Paladin marks the monster. So it can get punished or punished. There's no way out.
As I see, reaper's challenge weakens the monster immediately after he attacks. So if it decides to attack someone other than the swordmage, the damage is already compromised (is it correct by the rules)? Being that the case, the swordmage goes Full Def, gets a +2 on his already ridiculously high defenses (and sometimes, he will even use a stance, which makes it even more ridiculous) and his work is done.
Now, some people might argue: "it isn't a big deal. It's a daily" or something like that. But my group loves solo monsters. They're exciting bosses. The Dragons from Monster Vault are powerful and the Beholder is quite scary. Or was... Because now they keep getting perma-weakened.
So, my question is:
Does this power actually work like that?
And if so, does that last even after the swordmage is knocked out? And if so... Am I the only one who believes it to be that broken? Am I missing something?

A:

Yes, you are reading it correctly, but this is really just a worse, persistent mark. It's not overpowered for it's level, and is actually pretty easy to get around with the right monster. 
So here's how to do it:
Use area affects no less than every other round. Solos often have area effect powers, the power only requires that the swordmage be included in the attack, thus they should make area of effect attacks whenever possible.
Second, they should kill the swordmage. First if possible. This is fairly standard kind of thing to do, but it's just a good idea, focus fire on the swordmage and when he's dead, you don't have to worry about this.
Don't use solos solo. Yeah, it's nice theater for a solo to be alone on the battle field, but it's terrible tactics, and is vulnerable to just this kind of lockdown. Use minion (a bunch, or spawn them), lackeys, summons or henchmen to provide alternative sources of damage. 
Lastly, if you're using a monster that gets multiple turns per round, keep in mind that these effects only last to the end of that next turn (not the whole round), so you can attack someone else, be weakened, attack the swordmage again (or use your area effect), and end the weakening for your subsequent turns.
And I know you say you don't want to hear this, but L15 encounters are game breaking, it only really gets worse from here. Your monsters will have to be smarter and have better tactics. And if your players are using the same tactics over and over, it's probably time to switch up your style. For instance, a fake boss might be a start.

