2 hits? I took like 7 before I died, and I killed it with one clip of Alien Blaster. Shouldn't be too difficult unless you use a crappy gun like a brush gun or something, dont use any Guns based weapons because it has a shell that resists even AP rounds. Stick with Energy Weapons, like the Pulse Gun for an easy kill.
Just because you killed something with the Alien Blaster doesn't mean its not a difficult fight. I could grab the alien blaster and just walk into a Deathclaw nest with a Legendary Deathclaw and not get scratched because its stupidly overpowered (like its supposed to be). Alien blaster = easy mode so its really no accomplishment to kill anything with it.
Also calling the Brush gun weak when it has some of the best DPS in the entire game with the right ammo and perks means you obviously don't understand guns in this game.
Also the Giant Scorpion doesn't have a shell that resists AP rounds (nothing resists AP rounds) it just happens to have a DT higher than what AP-rounds deduct.
(Example: AP- Round is -15 DT but the Scorpion has 20 DT or 25 DT).
The only correct thing you said was to stick to energy weapons (aka Pulse Gun).
I used the FIBO on the Bloatfly, not the Robo scorp, I used Elijahs Advanced LAER and took it down with about 7-10 shots. My courier has 100 in both guns and laser weps so I can use any firearm out there with maximum lethalty.
I would disagree with the fact that you can use any firearm with maximum lethality (unless you happened to have picked up all relevant Gun AND Energy Weapon PERKS as well as raised both skills to 100).