Is thornmail worth it on Galio? I mean...if you get a really good ult off, you're doing extra damage to most of their team with the thornmail. It forces their ADC to wreck themselves. I would think thornmail is the only armor item you would need. If their AD is really giving you trouble, maybe add in a FH.
if you swap out thornmail for randuin's you get a better passive and an active while the passive may make it hard to max out the dmg from your ult, just remember you can use your active... keep em clumped even after ult and aoe the crap out of the team
that way you also can MAYBE get rid of warmogs and get deathcap
im a fan of deathcap on galio --- a lot of people are sheep so they won't believe a champion is strong until a pro says so