A good strategy for getting the “Level a charge” achievement

I believe you need to play through the entire game without shooting anyone to gain this achievement. If you mess up, you can reload your last checkpoint and go again but if you've passed another checkpoint since, you'll need to restart.

If you want to check as you go along, you can press the "Back" button and in the top-right of the screen it should say: "So far, you have not fired a shot at anyone". If you think you may have shot someone, hit the "Back" button and check this. If it's gone, you need to reload your last checkpoint. If it's there, you're good.

The bit with the convoy doesn't count as a shot and you can actually shoot guns in the game (at servers etc), as long as you don't shoot at anyone.

I can't give you a definite answer for comparisons between L4D1 and L4D2, but I have racked up hundreds of hours on both games and continue to play both daily. I'll give you my tips for each SI class, and perhaps it'll help you a bit.

Hunter

The obvious:

25 damage pounce is of course, wonderful. Some people are pretty good at bashing you out of the air, and it can be quite obvious when they hear you leaping. Practice makes perfect on this, after the hundreds of hours I have put in, I can land high damage pounces even on moving targets.

Pounce when people are stationery, or pouring a can. On scavenge this works better as people will shoot you and often hit the can, igniting it and causing fun.

You can bash hunters out the air just through practice, eventually you'll get the timing right.

The less obvious:

Most of my pounces are done by instinct. I don't point down to see where I am going, I just guess where they are. Surprisingly, I land quite a lot of pounces this way. By facing downwards to see where you're going, you'll often fall short.

If you hunter someone, surrounding people stagger back, and can be forced off ledges.

Hunter can run very quickly, if you're bashed off someone, it is often better to attempt to run away rather than crouch/pounce. In many situations you can dash behind a corner and pounce away.

If you miss, don't pointlessly leap onto a survivor, instead retreat and try again. It takes 20+ seconds to spawn, but only 5 to pounce away, onto a roof, and try again.

You can pounce off walls, so practice jumping between buildings to improve your height.

Pounce damage seems to be based on your height when you begin the pounce, not when you end it, so pouncing very high and landing on the same level does not do much damage.

If you pounce repeatedly against a wall you can distract people who get nervous from the sound.

Smoker

The obvious:

Best to pull people away from a group, or off a ledge. You can hold people still for a pounce or charge.

The less obvious:

Smoke people who are reloading to stop them spinning and shooting you. Smoke people off ledges and have your team distract. It only takes a few seconds of distraction/interrupting someone for smoker to reload and get the next guy. You can counter smoker with most melee weapons by immediately striking the tongue. Also by spinning and dropping (not throwing) a gas can onto the tongue.

There are many spots for smoker/hunter instant death pounce. It's worth researching these if you play with a friend, as there are many more than the obvious well known ones.

Smoker can instant kill or incap people who are walking up escalators as it can drag them up and over the side, and they won't grab on.

Running up to people and smoking them while standing right next to them will grab them without any chance of them bashing or escaping. This also bypasses the generator bug on no mercy rooftop scavenge.

You can smoke people next to ledges, and chargers standing on the ledge can grab them, allowing you to charge people off the map. There are many good smoker/charger combo points that people don't expect if you're working with someone else.

Charger

The obvious:

Charges people, knocks people back, has a very high damage punch that can hit multiple targets. Can kill people by charging them off the map, or by causing others to fly off the map.

The less obvious:

Charger can do more damage with a punch than by charging into a group and getting killed. Works well with boomer.

Charger can do ridiculous damage to someone who is smoked by punching them, incapping them in a few seconds.

The best way to connect a charge is to judge the player's skill throughout the game. If they have high skill, they're likely to be listening for the charger charge sound and dodge. If you adjust your charge accordingly, they will often dodge into your charger.

Always aim charger in front, charge earlier, etc.

There are many places where you can charge two people, and have the 2nd person go flying off the map. There are lots of less obvious places, such as the mall finale - instead of the obvious top of the stairs charge, you can hide in a side room and when two people run back with cans, you can charge them diagonally for an instant kill, etc.

Survivors in spit take less damage when huntered, jockeyed etc. which makes it less effective. However, survivors that are hit by a charger and go flying through the air continue to take regular damage. Charging and spitting in a confined spare therefore does a huge amount of damage, including if you spit into the place where survivors are landing.

Cautious survivors who think you're waiting at the top of a ladder will popup and go down to try and get the charger to waste their charge. If you stand along the top of the ledge and charge diagonally, you can in some cases hit the survivor by falling off the ledge and catching them half way up the ladder.

Exploding boomer can free a charger. Consider this if a charger has incapacitated someone as they can free the charger to move to another target.

It's very hard to charge someone who is getting up from being pounced, as in the video.

If someone grabs a survivor with a jockey or smoker, and you have the opportunity to charge them off the map, consider if you can do better by charging the person who comes to rescue them.

Don't charge at point blank range, as it will often bug and you will either not connect, or you'll just end up thumping them at close range.

Jump up and down when punching people as it throws off their aim. Charger takes more damage if shot in the head, so don't give them an easy target.

Honorable mentions:

Spitter - always spit in the center of the room, never in the corner. The radius of the spit is bigger than it appears on screen, and players will think they can escape by running to the edge of the room where it looks safe, which is often not the case. You can spit upwards and hit the roof, the spit will then fall down and you can stay hidden. Spit that bounces tends to spread out more, so try to bounce it off walls if possible. If you spit and then die in the right place, the modifier for spit damage increases even if they change from your spit spit to your death spit and it can inflict massive damage if you chain both together.

Boomer - Spawn and fire, don't run forward after you spawn. Aim upwards for increased range. Run as a ghost, jump, and spawn in mid air to get a speed boost that makes you go flying.

Jockey - Sucks, but if you incap someone near a ledge and time it just right, you can insta kill them as their incapped body will fall off. Jockey also refreshes its pounce very quickly if you pull someone off a ledge. Some windows/ledges cannot be hung onto, so it's possible to instant kill from some windows/ledges, especially if you have forward momentum.

Tank - You don't lose control if one or more survivors are in the safe room. This means your team can chip away / keep trying to get a good boomer, while you chill outside. Very effective on maps that have retardedly easy to camp safe rooms. Punching cars etc. is best. You can shoot the thrown rocks out of the air. Anything the tank can bash can also be destroyed by the grenade launcher, so it's good to note these as you can only see them as the tank. Always incap one survivor, then hide nearby. When your bar gets low, bash the incapped survivor.

If you play the game a lot as you say, you probably know most of these, but they're there for others. I didn't notice any difference in pouncing difficulty between the two games, they're both pretty tricky. There was a hunter bug in the first game that I don't know if it was patched, whereby if you tap crouch while standing (as a ghost) inside a car, it bugs you so that if you then spawn, you can sprint silently while crouched.