I played on Very Hard... You shoot deathclaws in the legs, when you cripple them then they can't leap attack you... then they can't surround you.
Sniper Rifle + Combat Shotgun with slug ammo.
I personally had more trouble with Cazadores because they are much harder to shoot.
Maybe you played before the DLC was released and the level cap was increased? Being over level 30 broke a lot of things (one of things things I mentioned above, that Deathclaws are actually considerably easier to kill before you get to be too high of a level). A few of the patches supposedly lowered the difficulty of the game too.
Either way, I'm pretty sure my Lanius fight was just bugged, it took me an incredible amount of console work to get the whole dam thing to play off correctly too. If you leave quests in your log too long in these games, they tend to get messed up.
I just loaded the game up vanilla (no mods) and messed around a little bit... I was able to kill a baby deathclaw at level 1 and a regular deathclaw at level 2 on very hard with a hunting rifle... I don't see what sort of outrageous claims I'm making, you can go do it yourself.