It may just be my philosophy, but APL really doesn't matter as far as putting the tension into the game. What makes the difference is wearing the PC's down. Rather than scale up a monster, add 2-3 more waves to the encounter, and watch people get wide-eyed as their resources run out.
One of the best and most frightening mods I ever NPC'd for was a fishbowl double-hooked mod at a halloween event. We were sent out (at an APL 17 game) as 15 body zombies, unintelligent and swinging 2s on a single short-sword length claw, with instructions to only hit moving targets. Infinite pops for 10 minutes, on a 5-count spawn. Why was it so much fun? Because we hammed it up. Pure Romero zombie goodness, down to the groans, staggers, and thousand yard stare. Why was it -scary-? Because we had almost 1-1 PC/NPC ratio, and even with level 15-20 PC's in the crowd, the eventual realization that slow 2's from a mob that just keeps coming will run you out of ability to keep taking damage was absolutely devastating.
Another great mod example is the endurance line battle. The PC's were blockading the front of a cabin while several rituals were cast inside, which would need roughly an hour and a half to complete if all went well. Of course, all did not go well. But this meant that ~15 PC's fought in a clearing against ~6 NPC's in waves for 3 hours. The NPCs were dark elf assassins, coming in with a couple globes of alchemy each, and spawning a new wave every 10 minutes or so while the Big Bad Drider stopped by every third wave to throw a few spells until we could chase him off, all the while taunting him madly. This was pretty epic, and scary as well given that the NPCs just kept coming, while our stores of healing and purification had -very- finite limits.
When you're in a situation where you have a number of new players like that, it really calls for a split mod hook. Send them off onto a scaled mod with a couple NPCs you can trust to take out a secondary target such as the enemy's reserves or supply stores while the big guys go after the fight scaled for them. It is very important, plotwise, to make sure that the lower level players -feel- and -know- that they were doing something important, and have somehow significantly affected the plot. If you don't have a split to hook them to, then at least give them some of the main enemies' backup to fight in the main battle, and let your NPCs know how to react. After all, a big bad guy isn't really going to consider that guy swinging 2's at him worth killing when there are really threatening targets on the field. That's what minions are for!