Kiting = Running away from your enemies, keeping them just out of reach, while you shoot them in the face. A sharpshooter can easily clear several waves by themselves doing this, as long as they have good situational awareness and level knowledge.
Zerking - It's a hard freaking job in KF. For any non-boss mob, you want to go for head shots (decaps.) It is by far a more efficient way to kill clots, bloats and gorefasts than wailing on their torso. Any mob that has been decapitated takes bonus damage from all sources...so your second swing as a zerker will usually kill a decapped zed.
You have to change your style for boss zeds though. Now your job isn't to kill them, but to tie them up so your other teammates can kill them. You want to get in their face ASAP (with good lines of fire from your team to the zed) then use your stun attack repeatedly. If you're doing it right, the boss zed will remain stunned the whole time. (Fleshpounds tend to not stay stunned as long though.) If you did it wrong, you'll get a stun or two off and then a skrake will stick his chain saw into your guts.
Being a berzerker is very, very hard until you reach ~Rank 4. Even with passive damage resistance, you get hit more often than anyone else and are in great danger of getting hit by multiple guys at once. A good berzerker set up pretty much requires a dedicated medic. You can get by without one, but you'll spend a lot of time running behind the lines to heal yourself, blocking people's shots ect....with a medic at your back, you can take a doorway and sit there the whole wave without moving. It also makes leveling for the medic pretty easy too.
But yeah. Berzerker takes longer to level and is more difficult to get leveled than any other class IMO. Even SS. Because while headshots take longer to get, your survival rate is about 100% higher.
As for Chainsaw vs. Katana, I prefer the Katana. The CS can be good under some circumstances, but you honestly want the quickness of the Katana stun over the much slower (but probably more damaging) chainsaw stun attack.