For hardcore, you just need to plan ahead. Getting into a tough fight, shoot a stimpack or eat food (if you have 40+ Survival)
before things get really hairy.
Being a sneaker/sniper helps me because I usually don't put myself in direct danger, but conservative play and thoughtful planning before combat starts helps a ton. Knowing you can lob a bomb, run them back into prepared minefields, or an alpha-strike from afar can make or break combat when you're outnumbered. I used a stealthboy to get in close and drop a bomb on a clustered group when I couldn't take them out with my pistols and rifles. Another time I waited for an NCR patrol to get slaughtered when they came across a Legion encampment before going in and finishing off the remaining Legion troops with sneak attacks. That one allowed me to take no negative karma and still complete a quest.
Companions are also your best friends. There are some really skilled and amazing ones in this game. The sniper and a certain gunslinger especially. Some of their perks are super useful, some not at all. They seem a little smarter too, they fight to their strengths. Find one that compliments your chosen focus.
The more time I spend on the Strip, the more complicated this game gets. The writing in New Vegas really puts FO3 to complete shame. I really have no idea what I'm going to do, there are just too many choices and paths to take. I've managed to get my character into quite the conundrum.