@Adriano gave a very good answer, but I'd like to add something else.
Ignoring your party setup for a second, I'd say any ranged and area / close attacks are a good idea to put pressure on the party. Artillery has very good attack bonuses and usually targets AC or REF (the latter of which is a weak spot for many defenders and leaders). Further, area / close attacks prevent many defenders from using their punishment ability if it does include them as one of the attack's targets. Of course, these type of attacks also allow the DM to exploit the terrain, attacking from higher areas or from behind portcullises or other areas the [melee] defender cannot easily reach.
Another idea would be to use monsters with damaging aura effects. Since the errata that allows damage from different auras to stack, monsters with big auras got much more dangerous. A similar idea works for swarms that make an automatic attack against each creature starting adjacent.
A simple group of 4 Chillborn Zombies[DDI] can deal an automatic 20 points of cold damage to each creatures within 2 squares, in addition to a basic attack that immobilizes and deals ongoing damage, bonus damage vs. immobilized creatures and a death burst attack that deals even more damage when a zombie finally dies. A melee centric level 6 party without a strong controller or ranged striker (e.g. fighter + melee warlord + melee rogue + melee bard) could have a pretty nasty encounter with this zombie group.
Bonus points are awarded for using some Rot Grub Swarms[DDI] below a trap door or something similar. With the errata to the swarm type that allows a creature to occupy a swarm's square, a creature can have a total of 9 swarms adjacent to it in a 3x3 shaft. This means that at the start of it's turn the creature takes
9 [# of swarms] * (5 [aura's base damage] + 8 [other adjacent swarms] * 2 [additional bonus damage]) = 9 * 21 = 189 damage. From 9 level 4 brutes. Not bad, I'd say. :D
Just joking! I'd hit any DM who actually used such a trap over the head with a DMG! :P
Last but not least I'd like to add a little anecdote. I've encountered a very similar problem in the past, and no matter how I made the monsters act the defender's player was whining. Did I make the monsters all gang up on the fighter, I was told that I was meta-gaming the monsters to deny the defender any chance to use his punishing mechanic. Did I make the monsters spread out and attack the other party members, I was told that I was meta-gaming the monsters to prevent the defender from being useful and soak up damage. Damned if you do, damned if you don't. *shrug*