There isn't really much to say, a few tips maybe...
- This game is balanced around you having a companion around, so keep 1 of them with you and only send the other one(s) on missions or whatever you do (to) with them.
- Your interrupt ability isn't there just for fun, use it to avoid being damaged by enemy skills that have a cast time.
- when facing an very strong opponent it's a good idea to enter the fight with near full (30) rage, playing annhiliation it makes your dots crit always wich heals you for quite a good amount acutally.
- Vette has an "AE-stance", use it, as you lack ae attacks yourself early one, just don't forget to change it back to Sniper Mode when facing single opponents.
- you can bring down extremly tough enemies alone when using your Cloak of Pain, Saber Ward + Call on the force, just keep your dots up, let your companions "tank" until you grab aggro again (takes a bit of practise to figure out how much time is needed).
-Always carry medpacs and use them, being a biochemist makes it easier/cheaper to get them and it's also a nice crew skill, easy to lv. (farm stuff with bioanalys while leveling and take Slicing for the money on the way, easily can get up to 150k creds minimum when you hit 25 and should purchase your speeder) and it's also pretty nice for endgame with stimpacks etc.
-Make sure your armor is up to date, we are limited to medium armor and do not have shields or high avoidance, so keep an eye on your armor rating. You will probaly get hurt more than ranged clases because you have to close down on your target and enemies are usually spread out a bit + you lack ae dmg and completely lack Crowd Control skills, which really sucks btw.
-if possible, try to get high crit rating/take gear with crit on it for questing when playing annihilation, as dotcrits heal you.
The basisc of dealing dmg as annihilation specced Marauder could be described as following;
Start your fight with charge + Deadly Saber, follwed by Battering Assault (you should always youse BA when it's ready and you wouldn't gain more rage than to reach the max) and Rupture and keep those dots running all the time. When facing multiple hostiles change DS to Smash.
Other than that just spend rage until you run out of it, depending on the cooldown of your rage using skills use Ravage, using your highest damaging skills first. Don't spend too much rage on Vicious Slash and end up wasting a gcd building rage before you can reapply your dots and/or use higher dmg skills.
Cloak of Pain has such a short CD that you should use it frequently when fighting a grp consisting of 1 or more strong opponents and a few weaker ones and always take the weaker ones out first, because they basically fall very fast, but apply your dots on the strong on for a bit of healing and because it's utterly wasted on weaker enemies who don't survive long enough for your dots to acutally do something.
Retaliation is also a handy skill, while a VS does somewhat more dmg it costs you a global CD, so Ret. + Assault is the better choice here, because the skills combined outdmg VS.
Don't think there is much more to say, maybe keep in mind that you can position mobs with line of sight (run behind stuff) if the enemy grp is spread and another thing is abusing Force Charge to get somewhere faster, you can use it to leap across gaps or jump on a higher lv/lower lv without taking fall dmg (at least I think that works) as long as there are enemies nearby.
So far, lv 24, worked pretty well for me, although I've only played Annihilation, can't tell you anything bout the other two specs, but it's fine, a Bounty Hounter might kill stuff faster, but he probably has to vent heat somewhat often instead of regen health, so it does not really make them faster... just less risky, you can die pretty quickly when you mess up as Marauder (missing all the stuns etc some other clsses have, an inquisitor should probably be able to solo some grpquests easily, having heal abilities, CC and still a decent dmg ouput almost as high as ours).
Marauder is definetly one of the harder classes to level up and to pvp with, we don't have heals (would've been so much better if dots just healed you right away with an ICD), no defensive mechanics beyond heavy cooldown Saber Ward and absolutely no stuns or at least CC cpells like the whirlwindthingy from the inquis or the no cd ae blastshot knocking down enemies longer than our Smash "stuns" them, but it's doable and not as bad as going through hell or having to watch the entire Twilight Saga (might be the same thing though).