I've been DMing for over 20 years. I've had this problem before. I've also done a good deal of adventure design, so I have a few tips that might help:
1. ELs: You need to tweak your ELs obviously. Parties with combat optimized PCs need to be treated at 1 level higher at lower levels and 2 levels higher at higher levels. PLUS on top of that you have double the number of PCs. That really breaks the system, IMHO. You need to add +2 to +3 just for that. See below for more thoughts.
2. Numbers: How you structure your encounters needs to change. You need to outnumber the party. So if you want an EL 14 encounter, dont use 1 CR 14 monster or 2 CR 12s or whatever. Use more CR 10s with a bunch of smaller minion monsters. With 8 PCs this is really a numbers game. If you have too few oponents, they can gang up and really do a sick amount of damage to your one big bad guy. So the video game "boss monster" idea is out with that party.
3. A Co-DM: The party gets the most out of its abilities because they have played their PCs over and over and grown with them. They know the ins and outs of their powers and abilites and items and know when to use what and what each otehr does well. And each player only has to run one (or two) PCs. They can focus on their individual action. Tehy have time to think. They have collective minds working on the problem. You, on the other hand, just created a batch of high level NPCs or monsters (probably with some character levels or other cool abilities). You have to run all of them. You arent as familiar with them as your players are with their characters. You dont have as much time to plan as they do. The solution: a "monster player" or a "co-DM." Get another person who is a good player to just run the monsters in the encounters. Give them them NPCs ahead of time and let that person tweak the NPCs out. If there are a few big NPCs, split them and both of you run them. Or if there are only a few but a lot of minion monsters, let the "monster player" run the NPCs and you run the minion monsters. I 100% GUARANTEE THIS WILL CHANGE THE WAY YOUR ENCOUNTERS GO. In fact, you might even try this solution first before you monkey with anything else. Dont feel bad or inadequate that you cant do it yourself. With 8 PCs, you cant. Heck, I ran a whole campaign this way and it was great. It was a bit boring at times for the monster player, but I set it up that he and I swithced off nights of DMing and when not playing the monsters he was tweaking the next night's adventure. Often times he just flat helped me DM. I did the same for him. It was great. Better roleplaying, too.
4. Story Solution: What you really have is a group that needs to be retired soon. But players HATE to give up their powerful PCs that they worked so hard to build. So here is something I did in the same circumstance. I had the players all make new 1st or 2nd level PCs. I told to bring both the low level PCs and the high level PCs for any game session as I might choose to run one or the other on any particular night (usually I alternated). I made sure to stop the nights adventure at a cliffhanger--usually right before a combat. "OK, thats where we are stopping for the night." I;d get cries of "No!" But it helped them want to play again and built tension when I ran the other group. This particularly works with teh lower level PCs. HERE IS THE SECRET: Set the lower level PCs in the same campaign world, but advance the clock about 100 years. Get the high level PCs to go against a legendary foe or build or quest for a legendary magic item. Let your low level group know the legend of the old group, but have the end of the story be lost. For example, "the crimson band set out against the great dragon Anargalon, and neither was seen again. It is said the paladin, Bannor, fell in that combat, but no one knows the truth." Now you have created mystery. Have the low level PCs travel to the tomb of one of the current high level PCs to retrieve one of his past items of power. Then, you actually can play out the end of the first party while the new party is hooked into that story as a legend. It really plays out well, though it takes some subtelty on your part. Basically, if you want to take away characters, you need to make it COOL! Players will always do something that makes their characters "cool" even if it means they dont get to play them again.
Good luck.
Clark
