With regards to anti-deathclaw (or similar high end enemies).
If you're like me, you dump most of your points into intelligence and leave one spare point for something else (in my case, agility). This means that a great many weapons are simply unusable for much of the game unless you waste perks on SPECIAL upgrades or resort to taking a lot of drugs.
Thus, for most of the game, the upper limit of my firepower was a sniper rifle. That might not seem like much, but it turns out that, with the right perk selection, maxed guns skills and the right kind of ammunition, there isn't much that will shrug off a single round to the head. When I was tasked with collecting the eggs of various creatures for example, I assumed I'd have a problem when they inevitably sent me after things like Deathclaws but even in this case I had little to worry about as the Deathclaw Mother crumpled after a single shot.
Sure, the Anti-Material rifle with the proper ammo and perk selection does sufficient damage to kill anything in one shot ( ((121 * 1.15 base damage with match grade ammo) + (83 * 1.5 (critical hit with better critical)) * 2 (headshot) * 1.25 (psycho)) = ~660 damage in a single shot (for anything with a DT < 65 (which is to say, everything) compared to the sniper rifle's 405 (for anything with a DT < 35 (which is to say, most things), but the truth of the matter is even at level 30 you won't really run into things that won't be killed by the sniper rifle anyhow. Better still, the sniper rifle has a x5 critical chance multiplier (meaning even if you aren't going to get the guaranteed critical from a sneak attack you're x5 times more likely. In a reasonable scenario (5 luck, recon beret, finesse) you will regularly get a critical hit (5% chance from luck + 5% chance from beret + 5% chance from finesse * 5 = 60%) compared to the Anti-Material Rifle's 15% chance. And the sniper rifle can be silenced meaning you can actually take down more than one or two targets before you lose your sneak attack bonus!