The Desert Eagle is easily the best sidearm for the job here. You will unlock it after Level 4 Assault ACES (120 Points). Don't be afraid to use it, usually you can kill with one shot - using the laser sight can help a great deal in tight situations.
Alternatively, you could wait until CQB Level 8 for the Glock 18. This pistol is fully automatic, albeit, quite inaccurate, yet you can still make quick kills with it.
100 kills means cumulative, not all in one match, balance your use of the pistol with other weapons because it will help you get the other weapon achievements.