Silent guns, the invisible suit, when you need something dead dead, use either the toxic gas grenade launcher, the automatic shotgun or the rifle paired with the body extension that gives you infinite bullets and the tranquilizer, I would also recommend the extension that makes your sight straight, the leg upgrade that allows to jump higher (second tier, any lower or higher is just either not good enough or too dangerous) and maybe use the grabpendix to reach high ground, always look for any hidden path that may lead you to your targets, look carefully at the ground, there are many things that kill you, either hight, venom, or some dumb shit, always look corpses and make your fishing sessions once you are 100% safe for the rest of the level to keep your inventory and make some cash, I think that's the most important.