I ran a short 3.5 campaign once with a warlock, a halfling paladin with a sling, and a ranger with the archer build. The party wasn't just lacking a "meat-shield"... it had no melee characters at all! This created some harrowing battles. I had never seen so much movement in 3.5 combat before. Eventually I gave the Paladin an NPC gnomish squire to be her "shield bearer" to soak up some hits, but honestly? All this did was make battles less interesting.
You phrase this as a "dilemma" but I honestly see it as an "opportunity"! Combats with this party will be brief. Protracted battles will almost invariable favor their opponents. It's been mentioned that reconnaissance and planning will be your player's best friends. Give them the ability to set the terms of engagement most of the time. They'll get to feel awesome, striking from the shadows quickly. It seems like your players are pretty down with intrigue, so it's hard to imagine them not being cool with this style of gameplay.
This also has the added benefit to you, the DM, in terms of creating challenging encounters. You don't have to mess too much with the fiddly mechanical bits like giving enemies more HP or higher attack numbers to make a fight more difficult. You just have to take away their advantages. If planning is their biggest asset, have the enemies catch them unaware. Or let them plan, but make it more difficult for them. This party will like wide-open spaces for manevourability. Make them plan to take someone down in a narrow space. They like combat that can be done quickly. Put them in a position of defense; where you can dictate the length of the encounter.
You've got a big enough party with enough of an emphasis in manevourability that you can also play around with the scope and size of battles. Your party can't be tethered to a "Defender" character if they don't have one in the party. Think about how to split apart objectives, and how your party might split themselves up to tackle them accordingly. Add in some sort of ticking bomb (literal or metaphorical) and you've got a recipe for an intense encounter regardless of how personally threatened the PCs might get.
This is an opportunity for you as a DM to make the terms of combat almost if not more interesting than the baddies you're throwing at them. Where they're fighting and how they're fighting get to take the spotlight over what they're fighting. Throw off the tyranny of the "stand still and hide behind/hack away at the meat shield" paradigm. Go crazy! They aren't always going to be slam dunks, but you have a golden opportunity for experimenting with your encounter design here. Embrace it!