ok, beating New vegas with a bb gun and no defense was easy compared to what i did recently. While difficult, playing through Fallout:New Vegas with those conditions is not nearly as hard as playing Half Life Opposing force on Hard difficulty (in my opinion.) Opposing force, if you don't know, was an expansion for Half-Life made by Gearbox where you took control of one of the HECU marines in Half Life who was left behind in black mesa when they extracted. Ramping up the difficulty to Hard makes the enemies do much more damage, and have much more health, while you get less health from anything that can give you health. Opposing Force introduced an entire new type of Alien species, Race X, which was much more lethal than the normal aliens from Half-Life. Shock Troopers were the most annoying out of all of Race X, as they shot fast moving electric projectiles, shredding armor and doing 10 damage per hit, with a high rate of fire. On death they drop a Shock Roach which is a monster that can be picked up as a weapon, but if you already have it, it acts like a normal headcrab.
In the game, there were also Human enemies, mostly Black Ops. Male Black ops threw grenades all over the place, rushed you, shot very accurately, and did a ton of damage, taking a ton of damage to kill them. Female Black ops had about the same amount of health, except they went invisible and shot you with a highly damaging silenced pistol, mostly focusing on taking you out from range.
Overall, Hard mode on Opposing force was hell. With Health and Armor being scarce throughout the game, and not very helpful (10 health per medkit) and tough enemies that would swarm you in large groups, I was constantly dying because my autosave had left in at a point where I had 5 health. If you play through opposing force, use the desert eagle, explosives, the sniper rifle, the displacement cannon, the grub gun, the shock roach, and most importantly the M249.
(i just wasted so much time writing this fml)