I strongly suggest you use the rules for creature types from 3.5 D&D in Conan. If you don't have these books you can download a free SRD with the relevant rules. According to these rules only constructs and undead are immune to massive damage saves. A huge creature with a bunch of hp will usually have a good DV, DR, and Fort save. It won't be immune to massive damage saves, but at least it will be somewhat resistant. You could also use an optional rule from D&D that increases the massive damage threshold of larger creatures. Save DC's will remain the same, but larger creatures won't have to save as often.