To reduce the damage, a Shielding Swordmage should use an immediate interrupt action. Thus, he can use it only once/round. So, if you have 2+ monsters who are serious threat, you can always let monsters just ignore mark and attack much squishy PCs. The first hit may be reduced it's damage. But the second and the later hits in a round will cause full damage. -2 to attack? Accept it. That is expected penalty as almost all the parties have some defender(s).
And a Swordmage is not particularly good at marking multiple opponents at a time nor good at punishing multiple opponents/round. For monsters, ignoring a swordmage is not as problematic as, say, ignoring a Charisma-Din sprinkling Divine Sanctions.
A paragon swordmage can easily keep two creatures marked. If they both ignore the swordmage, and if the strikers in the party have reasonably good defenses, after the -2 penalty, one of them will miss a striker, and the other one's damage will be reduced. Ignoring a swordmage is really not the best idea. Actually the paladin sanctions are a bit more ignorable, because all he does is a little bit of extra damage (unless you have vulnerability radiant, then it's probably best not to ignore).
The way to deal with a party doing well with a defender is simply to increase the challenge for the entire party (not just for the defender). You let the party equalize itself, and make some decisions on when, where, and who takes the damage.