You're taking too long to get into P2. At a certain point in Phase 1 you need to stop whatever you are doing and focus burning a leg so you can push into P2 before Superheated - less than 5 minutes.
0 Superheated stacks going into the first stomp in P2 is a dead boss.
Begin your burn when Rhyolith has 10-20 armor stacks remaining (do NOT wait for 0 armor stacks, unless you are perfect at killing obsidiums; we couldn't kill them all), and at around 50-60% health (but hopefully lower). Bloodlust at 35-40% health. Any leftover sparks should be tanked. During this time just have all DPS on one leg only to make Rhyolith move in a circle and don't worry about new volcanoes.
Make sure Fragments are tanked on top of bosses legs, so that incoming obsidiums are dying to incidental AoE damage.
The boss is a timing fight, you need to plan your burn accordingly.