what is the best way to kill them

Radscorpions. The best way to kill them is with a Fatman.

Truthfully, there really is no "best" way to kill them. I use a combat knife running side to side avoiding their attacks most the time, although a Riot Shotgun, 10mm submachine Gun, or really any decent gun works as well. Also, an Anti-Material Rifle works as well. Or an Alien Blaster. Or a Grenade Machinegun.

^^^^ this, with Splash Damage, Demo Expert, and Esther with Big Kid ammo, your going to practically laugh at how they fly and how you feel like the Nuke-God of fallout

Dont shoot them in the legs. It wont slow them down but you will just end up doing less damage. Other than that, it depends on your difficulty and weapons load out. On very hard difficulty its not really possible to face Giant Radscorpians at the start, unless maybe if you have explosives skill.

AP rounds also work well against them.

I like to kill bugs with flamers....it seems appropriate.

I like to cripple the tail in VATS so at least I don't have to worry about getting stung.

I can wipe out giant radscorpions with the super sledge and only few hits playing on very hard level, need at least 45 - 50 Melee skill & super slam perk. Looks great when u c them being stunned and rolling over....lol...