This suggestion might not be quite what you have in mind if you don't want an instant TPK, but a "challenging" (APL+1) encounter can easily murder an L6 party using only level 1's.
Your budget for NPC's with player class levels is CR7 for a party with APL 6.
NPC's with PC class levels count towards CR at one step lower than their number of levels, for L1's this means they are CR 1/2, so you can have 14.
14 wizards - level 1, cat-fearing wizards..
Their WBL is 390gp each, or double this if your setting is "high fantasy" (20 point point buy). Either way, they can comfortably afford a level 3 scroll each. Let's say all Fireball for simplicity (you could go for a much more nasty mix).
The DC for them to cast successfully is a poxy (DC = scroll's caster level + 1), and they all have a +1 to the check as they are L1 casters - they need to roll a 4. Therefore, they each have a 17/20 = 85% chance, meaning you will get on average 12 off successfully. Make sure everything else about their build is geared towards extra initiative. Improved Initiative feat and the elven trait Warrior of old would be a good start giving +6.
Fireball cast from a scroll has a 520 foot range (CL3), so make sure all your mooks are 100-500 feet away from the party at the start of the encounter and really well spread out. They can just calmly encircle the entrance... or just spam the Fireballs through the room window come to think of it.
How well will the party deal with an alpha strike of 12 fireballs? My guess is not well.
Obviously the mooks can all focus fire plain old magic missile from spell slots at any smouldering survivor who lives and gets within 110 feet.
I'm not saying do this, it would be terrible GM'ing, I'm just pointing out that all the complex McGuyver shit in Tucker's Kobalds is pure fluff. Any CR appropriate encounter set up by a character who is not retarded (i.e. whoever decided to hire guards) with time to prepare can (and logically should) totally annihilate the party entirely within RAW, this method was simply the first that sprang to mind. Of course, logic is not always your friend when trying to tell a story.
I would also add that if you didn't have the defensive measures in place written down you before even told the party about the hotel, retconning anything "clever" after the fact is pure bullshit. If you making it up on the fly, you should roll the guards randomly or something.
In response to Drew's comment:
Drew: I guess the gist of my suggestion would be that you can mix in some cheap (in terms of CR) NPC's with level 3 and 4 scrolls, giving you access to a more interesting range of spells with higher DC's.
Confusion, Fear and Haste would probably be my top choices from the level 3 Wizard list. The first two would have a DC 15 will save, so there is a reasonable chance of someone failing.
Deeper Darkness from the cleric list could synergize well with L2 Barb guards with the Scent rage power. Make sure you read the rules on light levels carefully.
I'd probably go with two L2 Barbs, an L4 Barb guard commander, a couple L1 Clerics with L3 scrolls and a couple L1 wizards with L3 scrolls.
Hasted raging Barbs with Scent after dropping the PC's into a couple areas of supernatural darkness will make for a tough encounter, and also be somewhat interesting.
I would have the casters up on a balcony or something if possible, or on rooftops at a decent range if the encounter happens once the party get outside. Also remember that deeper darkness from a scroll will last 3 minutes and is cast on an object, so could comfortably be cast before combat starts if the guards know the PC's are coming or know where they are and move in to attack. You can shove it on the guard commander's armor. The encroaching wave of darkness spilling along the corridor as he and his underlings approach could be a nice descriptive element.
You can also legitimately have two follow up 'normal' encounters after the first 'challenging' one, which will hopefully burn off party resources.
Splitting the party is a good idea if possible, but dropping a portcullis by GM caveat seems a bit cheap since there really isn't anything the party can do about it. Having guards have keys to all the doors in the building and locking a door be a standard action seems reasonable (closing would be a move, the same as opening). If the party happen to get themselves on opposite sides of that big ballroom door and a guard can get to it and then next turn use a move to close it and a standard to lock it with the key, the party are unlikely to feel that you made something up not covered by the rules. You could use Bull Rush to good effect to reposition a PC or two to allow for this, and Bull's Strength cast from a scroll will mean those raging Barbs are hideously strong (and again it lasts 2 mins and so could be cast as the party approach or as the guards approach the party). Presumably the guards' cheif mage would have been Scrying the VIP, and should make an attempt to Scry one of the party immediately.