We all know that an alien tower is infinitely more valuable than a marine tower.
Let's exploit that weakness.
Beginning of the game--> listen for skulk **** and drop ammo to determine the hive.
1 IP, armory, armslab-> armor 1. 75 res.
Order all of your marines except one to cap the other side to go to one of the two usual resnode spots, try and kill the eggs or gorges before the RT goes up. Once armor 1 finishes (anywhere from 1:30 to 2 minutes into the game), hopefully you'll have at least two RTs and around 20-30 res. Invest those res into shotguns, drop those to your best marines, throw in a welder or two as well. Research weapons 1 after that, keep pressuring the RTs around the alien hive with the majority of your marines.
If you have a team that can shoot (not as good as 5+:1, but at least 4:3), hopefully you've just crippled the entire alien economy three minutes into the game.
Drop an observatory after weapons 1, research phase tech. Tell your marines to disperse into different parts of the map. Where there is an uncapped RT, drop a marine one in its place. If you see an alien RT somewhere else near the map, tell the marine to build a PG, and get marines flowing to that spot. Rinse and repeat until you're confident you've completely destroyed the alien economy to delay or prevent a hive or a 3 DC fade. Keep dropping shotguns, don't research weapons 2 (weapons 1 is already enough to kill a one hive skulk with the inner pentagon of shots).
For pubs, why does this work?
Easy, scream (well, not scream, but at least forcefully tell) your marines to go to the RT outside the alien hive. Most will obey, but there will still be one or two marines that go the other way, and they'll be your res cappers. Early shotguns boosts morale and make alien RT killing that much easier. After you research weapons 1, bar an obs and PG tech, all your res should be flowing into shotguns. Skulks are ridiculously easy to kill as are RTs, and the point is to cripple the alien economy from the get-go. Since your marines are out in force, it becomes increasingly difficult for even a 5 skulk pack to even kill one. With shotguns in the mix, it's incredibly hard.
RAHHH!!! BUFF SKULKS!