I'd do some reading over at the
Strategies & Tactics Sub-Forum. Take everything you read there with a grain of salt until you have either seen the poster in-game, or read enough of their posts (and the responses to those posts) to know what you think of that person's opinion.
Cheers!
P.S. When using a rifle against dretches, it has often been suggested that you remember to fire in short bursts (at least until you can be sure that every bullet is hitting its intended target). Spraying wildly wastes the limited ammo you can carry, meaning you will run out, or have to return to base for more, long before you've had a chance to reach the enemy base. It was suggested to me to fire in three-round bursts. I believe the rationale was something like this: each bullet does 8 (?) damage, and a dretch has 25 health (8 damage times three well-placed shots in one well-targeted burst equals 24 damage if all three bullets make contact). Most dretches will suffer some collateral damage or crossfire, therefore dying, or run away like frightened cockroaches when the kitchen light is turned on at 2 am. Anyways, it's good practice in not being too twitchy, and makes you think more about placing your shots.
A couple of things to remember: Dretch headbites (even sometimes when it seems like they didn't drop directly down from above) do 96 damage to an unhelmeted Human head. As soon as Stage 2 starts and you have the cash - PUT A HAT ON! Remember to look up. Find a buddy, and watch his back. Leave base with a group, or with a more experienced player. Play Aliens more, to learn the nasty evil tricks of the dretches. And spend one or two games just spectating the players who always kill you. Watch them from their viewpoint and learn how they did it. (Ask someone about ground headbites.)
Good luck, Tremulous is an awesome game.
