Personally, I'd say that HP are the less important stat when battling sea giants. (Mind, I've only killed about a half dozen of them, so I'm no expert.) I've had them kill me in one hit even when I have 200+ health, so the better solution is: Don't get hit! It's far more important to have good armor and (especially) high dexterity - armor will reduce the damage you take, but dexterity will help keep you from getting hit at all. Use whatever magical means you have to thoroughly boost your dexterity before you take one on, and (this is key) save your game before you do so! I still die about 40% of the time when I try and kill one.
It also helps to have high strength and good weapons, so that you can kill them slightly more quickly - but it's more important to make sure that you don't get killed first.
Finally, if one does hit you and not kill you instantly, heal fast - anything less than a healing potion is a waste of your time, and philters (aka "filters") are much better.
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"A scientist can discover a new star, but he cannot make one. He would have to ask an engineer to do it for him."