During Vasco's side quests, you'll eventually be asked to explore a ships wreckage. This Guardian is a pain in the bum but is still fairly easy to kill on Extreme Difficulty even.
I kill this monster at level 11. Make sure you have tons of bullets, at least 60, you need to destroy it's armour rating pretty quickly. This won't kill it. So, yoy need to use hit and run tactics.
Craft a resusitation potion at the workbench before the arena also.
When the creature is at around 1/4 - 1/2 health. use the potion. The creature tends to stick to you so Vasco and who ever else you took (I had Kurt with me) can pretty much do damage to it for a while.