what's the best way to kill the savage deathclaw at the museum of witchcraft ?, i tried using a missile launcher with a load of missiles, sadly no luck

I think I used my Gauss Rifle.

Not doing this quest too early helps...

Go for the soft underbelly of the beast? I haven't gotten there, so thanks for the warning on that booger.

A companion in power armor helps

I got the Overseer's rifle at Vault 81, awesome firepower once at max level

Using the terrain always helps. I used a small doorway and just kept popping in and out quickly popping off a shot or two before fleeing like a small child waving his arms in terror.

Take some shots at it then retreat back to the room you came through, the deathclaw won't go back there, you could try standing by the hole in the ground so that when he gets close simply drop down