Let's see, I've got a gold Tac .45 and am grinding through the B23R (so annoying) and am on 76 headshots with the Five Seven, so just about to get Kryptic Typhon.
I would run extended mags on the B23R when you start using it, because it chews through ammunition like crazy, while the Five Seven has the largest magazine of any pistol, so doesn't really need it.
My pistol class set up is:
1. Ghost + Lightweight
Lethal: Grenade (greater blast radius than Semtex)
Perk 1 Greed
Secondary: Pistol + either Long Barrel and/or FMJ.
I did all pistols except the executioner a long time ago. Every pistol so far had been very easy. The executioner was a battle I wasn't willing to do for about 2 months. I hated it because of so many reasons.
long reload time
However, over the last two nights I have 1 blood thirsty to go and I'm done. I changed everything about how I play and made this class setup
ghost / lighweight
dead silence / awareness
long barrel attachment & 1 shock charge OR concussion
I basically bated and used my shock charge to guard doorways / rooms or if I was watching the other side / door I used it as a warning device. The concussion I used to chase people down and catch them.
The hardest part at first was figuring out how to run around on the map and get in close to people. I can't tell you how many times I saw an enemy and just had to run away. The other tough thing was finding places on maps that I could lock down with a semi-high traffic rate.
Not the most fun weapon to diamond but the remarks you get from people with the executioner makes it worth it.