TBH lightning is probably the worst element against that boss, but it's still definitely possible. I think the issue is that you're attempting to do too many things. Sure, it takes extra damage from fire, but if you're putting fire on your magic attack, you're wanting magic damage for fire, physical damage (because it's still your main damage source at that stage), magic skill (because you kinda need it for lightning), and heal gems (to proc antidote gel). You've created a scenario in which you *can* use any gem, but aren't specifically good at using any of them.