Survival can get you a lobster shield, which is pretty good but I prefer the Androbot shield. More importantly, you can get healing glands from dragon enemies, or free meat and sushi. Also, you can use Science on defeated robot enemies. Most of the time you'll get spare parts that can be used for General Repair, but if you use Science on a defeated Androbot, you can get something to install in a mecha that gives +1 to the mecha fighting skill.
Robotics can be extremely powerful at high levels too. It's possible to build a sentient robot with stats in the high 20s to low 30s with enough skill. That's far, far better than anyone you can hire. Just build a robot using 8 mecha welding kits, a level 10 targeting computer and level 10 sensor. There are probably better combinations but that works well for me.
Deathwings are good throwing weapons because they're poisonous. But the best weapon for cleaning out sewers is the Toxin Grenade.
PV is composed of equipment, inventory, and installed cyberware. If you go the cyberware route, you'll get a lot less xp. Spirituality is generally better, since you can get a lot of free xp from shrines, and there's a shrine quest that gives bonuses to stats.
One of the best ways to get experience is to raise your Knowledge score really high, and then use the GARU serum. The GARU serum reduces your Knowledge and Charm, and increases your Body, Reflexes, and Speed. If you have high combat skills, you can drop all of your gear and punch mecha to death.
If you get the Badass trait, your combat skills improve when you're in a bad mood, so you don't have to worry too much about getting expensive food and the like.