User blog comment:Mathmagician/Hard Mode Bosses Solo Walkthrough/@comment-4698557-20111227001331/@comment-4674838-20111227003425

Hellstone bricks (also meteorite, hellstone, etc.) cause fire damage when you walk on them. As I explained, that mechanic can be intentionally exploited so that you're getting hurt by fire damage sometimes instead of getting hit by bosses. Which would you rather take: 1 damage from fire? Or 40 damage from the Twins? I'd personally rather take 1 damage.

75 defense is only achievable with good +def bonuses on your accessories, which may require a lot of reforging. However, you'll notice that I didn't say that you need 75 defense to fight the Twins (not all 3 bosses). I just said that IF you have that much or more, then you can use the strategy I outlined minus the heart statues and still win (I've done it over a dozen times) if you're soloing just the Twins. Of course, you can still fight the Twins with less defense (50 will work, for example) using the strategy I outlined--you'll just be reliant on 1 or more heart statues for healing.