One good way to have them squeeze their brains to come out of an encounter is to make them feel what I call "the burden of leveling".
The further up you go in level, as a character, the less "commonplace" you are. Your group is around level 10, it's fairly reasonable, in my opinion, to think they would be at least somewhat known, and as you grow famous, so do your tactics.
The more your tactics are known, the better anyone can prepare in order to nullify them.
In a game I DM'd some time ago, the party came across a particularly nasty demon and the party necromancer absolutely squashed that thing, unleashing a barrage of spells that left it at around 3 in both strength and dexterity, several negative levels and no way to restore them. He escaped, observed their tactics and when he returned he had noted that most of said necromancer's most dangerous spells were ray spells, so he arrived with a Contingent Ray Deflection in store. Needless to say, the entire encounter was significantly more challenging than the preceding one.

As said in earlier posts, things like grease can really screw up anyone without at least 5 ranks in balance (as they'll automatically lose their dex bonus, so welcome sneak attack), and I would add glitterdust to outline the beguiler (first thing I'd do in the fight, to avoid losing the bugger to an invisibility spell later).
Also remember that a creature in a bank of fog treats any creature more than 5ft away as having total concealment; you can't target creatures with total concealment, if I'm not mistaken, and most spells in the beguiler would be therefore useless. Throw your beguiler in a bank of Solid Fog and you've removed him from combat for at least a couple of rounds, in addition to blocking a part of the terrain.

Terrain disadvantages, as pointed out by kantolin, are a good thing to exploit. I'm willing to bet both the warforged and the paladin wouldn't fare too well if thrown in a deep body of water, something that, at least in the warforged fighter's case, I think wouldn't be too hard to accomplish with a well-placed teleport.
Assuming both fighter and paladin are using magic enhancements and/or spells to pump up their AC, have a spellcaster ready with a handful of dispels, which should remove active spells and shut down their items, at least temporarily.

If you're a fan of ambushes, I would suggest some benign transposition shenanigans using trained animals. One moment a group of rats/ducks/any tiny animal is waddling along, scurrying between the party members, the next moment said party members marvel at how the cute and tiny animals have grown tall and are stabbing them in painful places.


On a more general note, you mentioned that "in two rounds" the warforged goes up to 36 in AC, I assume through the aforementioned buffs. A good start could be having foes attack in pairs, using one to mainly keep the character occupied and prevent that extensive buffing while the other attacks.

I have the feeling this post is more inarticulate than I thought it was, lack of sleep must be making me dumber -_-