Bit of advice for the actual fight:
You have three characters to beat Gargos but you have to remember that he has three health bars total to destroy (he always pops back up with a "regen" each time you fight him, even if you made him use it before).
What I would suggest, with keeping this in mind, is buff your team with artifacts, guardians and in-fight items and arrange them with the plan of your first character is just taking out his first health bar. Teammates 2 & 3 are the ones you intend to kill him with. Your first character should be someone you feel comfortable with but isn't your best. Give them a Snake Guardian, if you have one, and then go in on him just to take out the first bar. If you do, then just try to get the second as low as you can without killing him. The reason being is that the regen that Gargos does triggers every fight. So every time you take out his second health bar, he pops up in Stoneskin and begins regenerating health. This is why with your first character, that isn't your best but reliable, you are just aiming to hurt him and not kill him. After this, your last two characters (and presumably your better) just have to take down, ideally, his remaining second bar and whatever he regens on the third.
Also, his regen is pretty easy to exploit. As soon as he pops up and begins getting health, he is rendered. So plan for this by getting right up next to him and doing a heavy, grounded normal into a special move. Once he takes damage, the Gargos AI will stop gaining health. This allows you to keep his regen to only about 30% instead of a full bar. The Gargos AI will also use his Instinct Break 9/10 after you catch him regenerating with a special. So heavy into special into block. This will bait his break, ending his instinct and leaving him heavily punishable to end the fight.
So, that's about all I can suggest. Good luck!