Are there any tips that you guys can give to a FTL beginner like me? I feel like I do pretty well until I hit an enemy ship halfway through that I have no chance of beating. I'm playing on easy, too.
- Prioritize defensive upgrades in the early going. Shields and engine upgrades increase your survivability, and in the long run you're saving scrap that you'd spend on repairs by not taking the damage in the first place. Early investment in defense means more money invested into real ship upgrades by the time you reach the end of the game.
- You get more scrap if you can wipe out the crew of a ship without destroying it. Boarding teams, fire of all sorts, and the relatively rare anti-bio beam are the main ways to accomplish this, but there are some pretty esoteric ways like locking down the oxygen systems of an enemy ship with ion weapons that you can also fall back on. Getting together a reliable capturing method means more scrap, which means a better shot at winning.
- Float some scrap for most of the early game in case you find a major game-changing item for sale in a shop. Look for things like a Weapon Pre-igniter (all your weapons start fully charged after jumping) a Crew Teleporter (to get a boarding team built up for that delicious capture scrap), or one of the primo weapons (Burst Laser II and Ion Blast II were my traditional "buy immediately" tier pre-AE, some of the new weapons are mighty nice, too).
A lot of the game is just understanding what all the various systems and weapons are useful for and how to most effectively play the hand you're dealt. Most of this knowledge comes through playing, but hopefully that advice helps you enough to start accumulating that experience.
How do you get the final Lanius achievement? I played pretty much an entire run with oxygen at 19% but I didn't get the unlock. Can it never go above 20%? A few times, I got hacked and the doors closed so the oxygen went back up. Can't really do shit about that...
You can't ever let the O2 levels go above 20% from the time you make your first jump. If your doors get hacked you'll need to shut down oxygen production to stop the levels from rising.
When I got the achievement I basically just vented the entire ship except for the piloting controls. I got lucky enough to pick up another Lanius crew member so I had Lanius crew on weapons, shields, and engines with my sole non-Lanius crew member handling piloting duties. Towards the end I also picked up a mantis who spent the rest of the run sitting in the cockpit with my human pilot.
I like to think he didn't resent having to spend the entire duration of his tour of duty occupying the sole oxygenated square remaining on the ship.
Sometimes I power if down willingly to power weapons / shield. "Imma finish this quickly and then turn the power back on". Yeah, right 
