Piloting (IS) Lights
This is still somewhat of a WIP, I'm sure I'll think of more stuff to add.
First, a brief overview of the roles mechs take on in MWO/Battletech:
Scout: Scouts find the enemy and relay data to their allies. They generally have poor firepower, high speed, and low armor. Lights and some mediums (Cicada) fill this role. That said, you do NOT want to have pure Scouts in MWO. The maps are too small, the movement of the enemy too predictable, and the unrestricted targeting caused by the lack of a true C3 system makes them useless. If dropping in a group with some missile boats, then bringing narc/tag can be viable, but you must still have enough weapons of your own to be effective.
Striker: Strikers are fast mechs with high short/medium ranged firepower. The Firestarters and Jenners are Strikers. You generally want to run in a wolfpack, and use hit-and-run tactics to leg enemies one at a time. Focus on disabling enemy mechs and sowing confusion, less so on killing. Taking a hunchback's hunch or anything's leg generally takes that mech out of the fight, while finishing the mech off would take more time, heat, and ammo. As a striker you live for the words "component destruction".
Sniper: Snipers are long-raged fire support mechs. The Raven 3L, Cicada 3M, Thunderbolt 9S, Hunchback Grid Iron, KitFox, and similar mechs with ER Large Lasers, or ER PPCs or Gauss rifles fill this role. Snipers need to stay unobtrusive, firing a few shots at one flank, then repositioning before the enemy can arrive and hitting from another angle.
Brawler: Brawlers use heavy armour and heavy weapons to play rock-em-sock-em robots with the enemy. If you think you should brawl in a light you should probably lay off the cocaine for a while. Supporting your team's brawlers is important, they tend to make up the core of the "deathball". If they die, and the enemy didn't, you're in for a very hard game.
Skirmisher: The medium/heavy equivalent of Strikers. Skirmishers directly engage, but use their manoeuvrability to avoid taking too much return damage. The Thunderbolt TDR-5SS is a skirmisher. Engaging the enemy as the skirmishers engage, but from a different angle, can be extremely effective.
Missile Boat: Missile boats carry enough LRMs to deal significant damage to the enemy from behind cover. They require the assistance of scouts to be truly effective. Light mechs cannot carry enough launchers to be effective in the role, but can assist by NARCing targets. Many missile boat pilots will want you to "hold locks" on the enemy. Do not do so at the expense of being hit. Keep moving, keep harassing, NARC if you brought one, but most importantly don't die. NEVER stand still to keep a TAG on target. If the LRM Chuckers whine at you, ignore them. Going against missile boats, if you ever notice one that didn't bring backup lasers, kill it. If you see a light on your team trying to fill this role, take the 10k cbill penalty and put it out of its misery. It's the merciful option.
Juggernaut: Juggernauts die quickly to organized teams. They're big, slow assaults like the Atlas. Armour in MWO isn't as effective as in tabletop, a Juggernaut walking up to break an enemy firing line just gets cored and killed by people who can aim. Lights can try to support people playing this role, but it's better to ignore the village idiot and let him distract the enemy for a few seconds before he dies. When your team has a Juggernaut, try to use the time and distraction provided to get in a flanking position. Mechs which are juggernauts in tabletop work better as brawlers in MWO.
On to actually piloting:
NEVER RUN IN A STRAIGHT LINE WHILE VISIBLE TO THE ENEMY.
Speed is life. Speed only provides protection if anyone aiming at you can't hold you on target long enough to hit. Speed in a straight line makes it really easy for a bigger mech to "walk" their shots until they hit you.
When you turn, use jump jets to make the turn sharper. Pop the jump jets on, turn your legs, then hit the ground right as your legs finish turning and off you go. You want to perfect this so you don't spend any longer than needed in the air.
Some mechs (firestarters, spiders) have jumpjet animations that help you avoid damage. Hop these mechs: hit the jets, immediately drop off, then as you land hit them again, and repeat.
Try to avoid getting into a "circle of death". You'll take a lot of damage if they're halfway decent, so even if you win you lose the next fight. You also want to avoid this vs heavier mechs: each time you circle around a (Dire/Atlas/Tbr/etc) you pass through their field of view, letting them hit you. Use your speed to outmanoeuvre them and stay fully behind them, so they can't hit you.
When fighting another light, try to lead them towards your team. If someone is chasing you, run around cover, and once you have some space, stop just around a corner. When the enemy passes you, get on their six and stick there. If you're in the open, a quick half-circle followed by a sudden stop can work. Or, you can run straight for a short time, then hit "engine stop" (X by default) and jump. With luck, the enemy will pass under you. That said, try to avoid 1v1s. The enemy might have teamspeak. You don't want to get lead towards HIS team.
If you have a teammate in a light, alternating passes on an enemy can be devastating. Don't give the victim enough time to target either of you, and just take the poor bastard apart. Being in a dire and getting swarmed by locusts is like being nibbled to death by cats.
Hit and RUN. If you're trying to slug it out with someone, you failed. You're not a brawler. When you are running, jog slightly left and right continually, 1 step each way. With throttle decay on, you can also occasionally drop your speed a tiny bit and restart. This makes it incredibly difficult for an enemy to lead their shots and hit you.
Lasers are your friend, especially pulse lasers. They allow pinpoint damage, and don't cause cockpit shake. If your enemy (victim) has turned off in-game sound (some people do) or is listening to music (more people do that) they'll likely miss hearing the first volley hit. That makes it harder for them to know to turn around, and which way to turn. If they don't see the damage notification on screen you can sometimes kill them without ever seeing their front torso. If you're behind the enemy team, drop a UAV if you have one. Then hold target, wait for the LRM indicator, and shoot that guy just as the LRMs hit. This maximizes your chances of going unnoticed. LRM cockpit shake + sound = laser sound won't be heard.
Legs, Legs, Legs: When in doubt, hit the legs. When waiting for target info to pop up, hit the legs. Whatever weight class, aim at the legs. Of course if you know a mech is open CT aim there, same for pretty much any other part (except empty shield arms/torsos). But if you don't know that, leg them. Many pilots drop armour from their legs, so even if they don't drop much it's still below the theoretical max. They then tend to store ammo there, often on the theory that "no one legs a Dire!". This tends to end in an ammo explosion. Since ammo is consumed from the left leg before the right, try to take out the right leg first. While it's not as severe a problem for a Dire to lose a leg as it is for a Raven, it makes it much easier to stay behind them and finish the kill. It also keeps them from closing with your team, etc.
Going for the legs also helps prevent the enemy from spreading damage. When you aim for the legs the enemy turn to spread the damage to one of two parts: left leg or right leg. When you aim for a torso the enemy can spread the damage to 6 parts: both arms, both side torsos, and CT. Possibly also head. You can hit a leg from any angle, the torsos have front and back fields of fire.
MWO Ammo consumption order: CT -> LT -> RT -> LA -> RA -> LL -> RL -> HD So if you see a mech with two open side torsos, aim for the right one first. It might still have ammo inside! The "Target Info Gathering" module is good on any light with pinpoint damage, e.g. the FS9-A/H/S/E, JR7-F/K, any spider, locust, commando... Pretty much anything that isn't a Huginn or Oxide.
Try not to let the enemy lights get in behind your assaults. Likewise, try to get behind the enemy assaults. Which you choose will depend on your mech. A Huginn is going to be better at killing assaults than a FS9-A, but that firestarter will be better at duelling other lights.
Base defence/capture: As a light you'll be fast (minimum 130kph, average 150kph, topping out in the 160-170 range for locusts and commandos.) That speed means you can get back to defend a capture point quickly. When doing so, try to stop the cap, but stay alive. You want to distract the enemy and force them out of the cap point, or at least stay within yourself so that your team can arrive. Other times you'll get an opportunity to capture a base (or point) solo. If you can do so, it can disrupt the enemy team's coordination. Often some of them will turn around to try stopping you, allowing your team to push and destroy their front line.
Bring an arty or air strike, every match. If you manage to drop it on a clump of enemy slow-movers it will pay for itself. UAVs are nice for scouting, but less important overall than having a strike. Know the range of your strikes, and don't hesitate to drop one to deter a chasing mech. Drop it far enough ahead that you'll just be clearing the range when it hits. Whatever is chasing you will either break off or get hit. That's not as likely to pay for itself as hitting a bunch of assaults, but if it lets you live to get more kills then it's worth it.
Bring Radar Deprivation. Every light needs it. Then don't bring AMS, it only kills 3 LRMs per volley, and fewer streaks/srms. It can kill a narc, but that's a very niche use. The only exception to this is in organized play when you know the enemy team will have lots of LRM boats. Then you can bring a dual or triple ams light, such as an FS9-S or an ECM kitfox.
Locust: The locust is very, very fragile. You will die if an enemy mech so much as sneezes in your direction. Fortunately, it's very fast, though without jumpjets it's not that manoeuvrable. Getting people to chase you while your team kills them is your best bet in a locust. While that's fun, it relies on the enemy being stupid and is therefore a poor choice for competitive play. The main "serious" use of the locust is to allow one to take two 100-ton assaults in a CW dropdeck. The Pirate's Bane is the best of the bunch, take ECM, 4xML, XL190. Don't take the machine guns on the PB, their unfortunate placement means the muzzle flash will cover almost half your screen when firing. An extra pair of heatsinks is more useful anyway.
Commando: The commando is harder to hit than the locust, is more manoeuvrable, and has ridiculously flexible arms. It's also the fastest mech type in the game. The COM-2D sports ECM, and the Death's Knell puts out respectable damage for its size.
Spider: The spider is notorious for having the buggiest hitboxes of any mech. Unlike the previous two they can take jumpjets. The SDR-5D is the best of the bunch with its ECM, the rest simply lack the firepower to compensate for their lowered utility.
Firestarter: The firestarters are some of the best lights in the game. See the Master Guide. While not at all competitive, 7 medium pulse lasers and an XL255 on an FS9-S is a ridiculously fun build. It takes 2-3 shots to the rear CT to kill the average Direwolf. You get 3 shots before overheating.
Jenner: Good hitboxes, good speed, great firepower... and out-quirked by the firestarters. Jenners can poke more easily due to their high mounts, and the Oxide is one of the best hit-and-run mechs around, but overall they're outclassed by the firestarters.
Ravens: Ravens have big legs. Raven legs are easy to shoot. Don't let the enemy shoot your legs. The 3L with 2 ER Large Lasers is the best of the bunch, able to stay out of range and use the high-mounted lasers to poke the enemy down. The Huginn is one of the highest DPS lights in the game, handily outdoing the FS9-A. That said, the damage is more spread, which reduces its effectiveness.
Cicada: The cicadas are lights, dammit! The 3M is much like a Raven 3L with slightly worse range but better damage output.
Very brief overview of the clan lights:
Mist Lynx: It's a worse spider.
Adder: It's a medium.
Kit Fox: It's a medium, or a worse Raven 3L.