Right, so some basics. Homebrew campaign with 6 party members (stared with three and kept growing). Party is Lvl 12.
First character is: Fighter 9/Barbarian 1/Bear Warrior 2.Armor and sword enchanted to all hell (basically every penny he hasn't spent on potions) Sells everything to get more enchants. Has Robilands Gamble and Karmic Strike.
Second Character: Barbarian 10/Peregrine Runner 2. Low AC but puts everything he has into his two swords. Avg dmg per swing is 30+. Kills every thing in sight.
Third Character: Ultimate Monk build (He didn't know anything about D&D and looked up classes. Found it before I did and by the time I knew about it he was too far in to stop him.) Can do upwards of 100+ DMG as it's meant to.
These are my, so called, problem children. The rest are a standard Monk, Wu-Gen (Fire) and a Rogue turned Shadow Thief (homebrew class because Rogue prestige classes suck wihotu lvls in magic user). The problem comes in because we are in a city-based campaign, tracking down a black market ring. So not a lot of monsters there unless they get into like the exotic animal pens or something. So, tossing everything the DMG has at them with hoards of randomly generated NPC enemies. I don't know enough about being a DM to do this effectively so I'm coming to you guys. If I throw out similarly-leveled NPC's they cut right through them as if they were paper. If I throw out high-level NPC's they gang up on them, take a little more damage but still kill everything in their path. I've even tried pitting them against upwards of 20+ low-end enemies at once, but that just prolongs the battle. Nothing can get through the tanks AC except spellcasters. Spellcasters are quickly dispatched first by the pair of high-speed monks, the Wu-Gen magic, or the Rogue's enchated bow. Tank and Barbarian cut a swath through hoards of foes.
Sure, they're having fun because they feel like gods. But they're getting tired of the hoards of weak enemies. The only thing to ever kill a party member was a Lv 15 Assassin they didn't know about. Invisible, with a heavily enchanted scythe. Got a lucky sneak attack in on a old teammate. Critical and nearly max damage on all rolls instakill from nearly full health. Ragequit. Next closest was the Tank taking 4 spells to the chest as he told a ringleader to, "Go suck off his manservant to feel better about his men getting wiped out." Not eloquent but enough to piss off a man on his high horse.
I don't know what to do anymore. If I super spec out enemies with high-end magic equipment so they can mange to hurt anyone in the party, they just take it, pawn it or use it, and get stronger. If I use high-level opponents, they kill it, get mad EXP and get stronger faster. What am I doing wrong here?
tl;dr Party too strong, can't be slowed down. Combat is tedious because too easy. What do?
Edit for additional info:
Looking for NPC designs. In military structure think in terms of Sergeants and Warrant Officers. Race-Class build stuff with minions. Equipping them as you feel necessary. Party is up against a magic-heavy gang. Enchantments and buffs are not out of the question. Standard DMG build won't work for anything not a grunt. Anything 3.5 is cannon. I'm not looking to kill the party, or anyone in particular. But putting a dent in their ego and winning streak would be lovely. They might be new to D&D but they're big gamers. Anything obviously stronger than the rest will get targeted and gangbanged, tanks will relegate lower party members to crowd control. They understand strategy, but not necessarily game mechanics. I'll take whatever you have.
I'm not looking to have the forum do my work for me. But being pointed in the right direction for what I could use against the party would be a big help.