Oops, yeah that wasn't supposed to happen ...

Ok, so first thing is first ... sorry. After a bit of reflection I have decided that killing your entire party, um twice, was a bit harsh. On an up note, I did learn a couple things.
First off, when designing an encounter for a specific party, make sure you look for areas that your PC's may be weak in and try not to exploit it.
Second, don't use a monster that you think is cool because it can eat one of the PC's on its turn.
But I am getting ahead of myself.
The first encounter was more of a setup for the story than a challenge for Dave's party. I had the party in a random town checking out the marketplace when the lone wizard's tower, a mile or so away, basically exploded raining down fire and signifying round one.
As the town square emptied of people, a crew of 16 Legion Devil Grunts approached, broke ranks and attacked. While sixteen creatures look a bit scary walking up to the party, the fear quickly dissipated when Dave discovered that they were all minions. One well placed huge fireball took care of pretty much all of them. At this point we made a couple of jokes about how he is going to breeze through all three encounters.
We were horribly wrong. Cue Slapchop remix. Cause it is awesome.
The second encounter began fairly typically. A couple of monsters posing as injured humans lure the party in then change into horrible, um monsters. The monsters were as listed:

• 1 Spined Devil (Spinagon)- Very cool monster that can fly and fling spines(like people think porcupines can but actually can't) that catch fire and can give ongoing poison damage.
  
• 4 Infernal Armor Animus- Basically Hp bucket minions that give Hp to other monsters as they die. Pretty much useless unless you protect them like your virginity.
  
• 2 Succubusses? Succubi?- TPK machines. Yep I said it, Succubi are TPK MACHINES!!!!
  

This post is running on a bit so I'll stop for now, more soon.
In the meantime, put a Succubus in an encounter and let her do her thing. Then let me know what happened.