Here are few tips for really fast leveling without hexedit:
Play in solo-mode and use Faked Players mutator. Set Faked Players amount as 40 and start the game.
I would recommend leveling your sharpshooter first to level 6. Once you do that you can always start the map with Sharpshooter spawning with xbow (worth 600 once you'll sell it).
You should use beginner level because you only need headshots. When you use faked players mutator (40) you should get around 600 zeds during the first wave. That's pretty ok. Just use your 9 mm and make decent headshots. Run around and find ammo (which you'll find plenty in beginner level). MountainPass is a good map for leveling Sharpshooter.
Play in solo mode (normal) with faked players (40). Download DOOMed map and use it. In DOOMed map there's chainsaw hidden inside truck. Find it and use it against 800 zeds. Just find a narrow corridor. Once you die or complete wave 1, just restart the map again. This way you can get 1M damage / hour. So after 6 hours or so you'll have lv 6 berserker. Warning, this is boring as hell and will not make you a better zerk!
Once you have sharshooter in lv 6 start playing support specialist. Forget solo mode and start playing with other people. Always start with sharpshooter. After first wave sell xbow and change to support perk. Buy shotgun. Use maps with narrow corridors (like biotics, offices etc.). Save money and do not buy vest. Try not to die. Play in normal mode, do not use beginner, because there are not enough zeds and you don't get decent damage from killing them. Once you have money for AA12, sell shotgun and buy AA12. Save money. Once you have more money, buy Hunting Shotgun. Once you have Hunting and AA12, buy vest.
For fast support leveling use just shotgun (shotgun, AA12, Hunting). Eventhough you run out of ammo fast. Do not save ammo. Go for every kill! Try to get as many zeds in line as possible and then take them down. Use mainly AA12, once too many zeds too close, clear them with AA12. Use both barrels for Scraces and Fleshpounds.
Practise using testmap6 (downloadable). Practise against sc and fp.
Firebug leveling used to be very easy when weapons did not disappear between waves. You just used to buy 5 flames or so xD. Now it's a bit more difficult. In EvilSanta's Lair there is a spot where zeds can't reach you. Choose firebug and go to this place (once you'll have the firebug guns flame and mac10). This is boring as hell.
Play with others. Try to enjoy the game. When playing other perks you can carry ak47 or pullpup to kill stalkers to help leveling commando.
Host a game and ask your friend to join. Collect money during the first wave. Change to medic and buy MP5 and MP7. At the end of wave 2 leave 1 - 2 easy zeds and let your friend take all the damage. Heal your friend with mp5 and mp7.
When playing in public servers, just heal others even if you're not playing medic. You'll notice how fast medic level will increase.