Giant Frogs.
No, I'm not kidding. Giant Frogs are built specifically to swallow. When swallowed, the PC is stunned (so no actions), takes some ongoing damage, and cannot be targeted by any effects. (Save ends all).
To be honest, if you're going to be abusive, at least tell him to be abusive in a classy way.
A Jackalwere Deceiver (level 4 controller) and two Bravos (level 3 brutes) are all he needs. The Deceiver puts the warden unconscious, then the Bravo simply Coup de graces him. They auto-crit for 20+2d4 damage, so once you're done with the Warden you can progressively one shot off the rest of the party (as they will probably do enough damage each CdG on a non-primal character to easily hit their bloodied value).
But this is pretty pointless and the solution to PCs having strong options is not to simply ramp up abusing monsters.
My solution is to use creatures with auras - that deal automatic damage or effects. Get creative and use monsters that force the Warden to have to change tactics - like storm shards. The initial 1d6+4 or whatever blow won't phase him. When he has to move 4 squares from whatever he's trying to defend or suffer an automatic 3d6+6 damage - he'll sit up and take notice. Combine with enemies that slow or immobilize to make his life really interesting - and make all those HP count.
You could use fast moving skirmishers as well - deathjump spiders (they even suit your castle!). They can jump 10 squares without provoking AoOs, so they can get right into the pigeons (the other PCs) easily. The Warden can't do very much to them - so they can avoid him easily and take the battle where they want.
Traps and other things that the Warden cannot mark are a great idea as well. You need different tactics and skills to deal with a good solid trap or hazard - especially if these traps don't target the monsters whatsoever.
But try to avoid just abusing the more weirdly designed monsters in 4E like Giant Frogs and Jackalweres.
Contrary to Obryn, I believe you can find some monsters in MM1 or 2 that are top at damage dealing at low level. The better design shows mainly in consistency.
This is correct. Some of the best damage dealing creatures at low levels don't always come from MM3. They come from MM2 and some from MM1 - noting that the MM1 creatures are more just terrible combination's of features. For example Needlefang Drake swarms.
Examples of hard hitting low level monsters from MM2 are easy to find: Storm Shards for example I mentioned above are level 4 artillery. They have a damaging aura and combined with immobilizing friends they are incredible for damage - yet distinctly not abusive. Their ranged power lets them hit an enemy, then force it to move - often having to provoke OAs in a desperate situation - or suffer an automatic 3d6+6 damage. Any MM3 creature would be proud of that!
MM2 creatures - at least the heroic ones - are very competitive with MM3 and beyond creatures. It's only when their damage doesn't increase substantially through paragon/epic that MM2 creatures suffer a lot. At the same time, if you took a MM2 creature and boosted the damage - you'd have a far more competitive monster than a paragon/epic MM1 creature you did the same thing too. MM2 creatures have much better power design and so when you put the damage on top of them, they feel the most like creatures that could have been in more recent monster books. Coincidentally, due to the better power design combined with the fact heroic monster damage hasn't changed *that* much - MM2 heroic creatures stand alongside their later brethren with ease.
