I haven't experimented with all of the guns yet, but my strategy is:
1. I prefer to start with the Kimber because of its power. Hitbox detection is a bit sketchy in this game, especially when it comes to headshots, so the fact that the Kimber takes one less bullet to finish off enemies can be useful. The first few levels are pushovers though, so you can start with any pistol. The Glock is the second best with its Hi-cap mag (30 rounds!). As tempting as it may seem, DO NOT purchase the Glock 18 mod. Full auto is too inaccurate, and combined with the Glock's weak power, not so good. The M9 is pretty useless, but give it a try if you want. 2. This is the crucial part. Do not buy anything (except for the Heavy Belt, because it'll come in handy later) until you can buy the AK-47. 3. Once you get the AK-47, you will realize that it is a powerful weapon. At first, it suffers from problems of mobility and recoil, but with the foregrip and the sling, these are very much compensated for. 4. The sight is up to you, but I've noticed that the Aimpoint Sight is the easiest to use and generally the most accurate. 5. Buy the Heavy Vest. At about the point where you've bought all the attachments, you should invest in grenades for use in the one-technical waves. As soon as you see them pop out, prime your grenade and toss them at the location they're about to stop at. Prioritize shooting Rocketeers and crowd-controlling the infantry if they mass. If you're very gifted in aiming, you can shoot at the gas tank of the technical. With the AK the technical will blow up in one hit. 6. Start saving up those airstrikes. I would buy them as soon as you start getting about 10k, and buy them everytime you go over 10k. Do not overbuy. Repairs become very expensive, and you need a buffer of money to full heal your helicopter after each wave. 7. Time your airstrikes right. You should start using them when two (or more, if you're at that level) appear. Call them as soon as you see the first technical and don't shoot at it. Aim for the rocketeers above all, they will signify your doom if you cannot take them out. When the airstrike comes reload; you don't want to get caught with a half clip to take out the rest of your enemies and take further damage. If you have bought the vest, belt, and pouches, you have plenty of mags to burn. 8. The other guns are generally not worth it. The AK's power is comparable to the M14, but the M14's greatest downfall is that it does not have a grip. Therefore, I have found that its recoil is actually worse than that of the AK with a grip. The M14 is slightly more accurate, but when enemies start flooding your screen, power should not be your primary concern. The M14 may seem alluring, but it's a waste of money. Invest in airstrikes instead.
I got up to wave 39 with this setup, and perhaps I could have gone further if I hadn't decided (foolishly) to tackle three technicals with a grenade.
